* ventilator support of fio2 \>0·8 or peep \>20 cm h2o and ongoing use of more than two vasopressors for 2 or more hours with any agent at doses shown below in the supine position. * norepinephrine \>12 μg/min or 0.2 μg/kg per min * phenylephrine \>150 μg/min or 3 μg/kg per min * epinephrine \>10 ug/min or 0.2 μg/kg per min * vasopressin \>0.04 units/min * concurrent use of other investigational agents specifically for treatment of ards or inflammatory cytokines. (note: agents established to be efficacious and/or those used outside of formal trials are permitted as supportive data emerge) * known ineligibility for use of a ventilator for a minimum of 7 days, as judged by the institution's triage team * known allergy to msc components: fetal calf serum, human albumin or dmso * active invasive malignant disease requiring chemotherapy/radiation * other concurrent life-threatening disease (life expectancy \<6 months) or eligible for hospice care * known history of hiv infection on active treatment * females who are pregnant or breastfeeding * current mean arterial pressure (map) \<60 mmhg while on 2 or more vasopressors at above doses for more than 2 hours * history of any significant cardiac (myocardial infarction within 12 months of screening visit or unstable angina), chronic ongoing hepatic, or renal disease (grade 3 or higher); diagnosis of congestive heart failure with hypoxemia primarily due to decompensated heart failure; diagnosis of severe chronic obstructive pulmonary disease (copd) or interstitial lung disease requiring supplemental oxygen at home * concurrent diagnosis of diffuse alveolar hemorrhage * requiring continuous dialysis (unable to stop dialysis during study agent infusion)

* ventilator support of fio2 \>0·8 or peep \>20 cm h2o and ongoing use of more than two vasopressors for 2 or more hours with any agent at doses shown below in the supine position. * norepinephrine \>12 μg/min or 0.2 μg/kg per min * phenylephrine \>150 μg/min or 3 μg/kg per min * epinephrine \>10 ug/min or 0.2 μg/kg per min * vasopressin \>0.04 units/min * concurrent use of other investigational agents specifically for treatment of ards or inflammatory cytokines. (note: agents established to be efficacious and/or those used outside of formal trials are permitted as supportive data emerge) * known ineligibility for use of a ventilator for a minimum of 7 days, as judged by the institution's triage team * known allergy to msc components: fetal calf serum, human albumin or dmso * active invasive malignant disease requiring chemotherapy/radiation * other concurrent life-threatening disease (life expectancy \<6 months) or eligible for hospice care * known history of hiv infection on active treatment * females who are pregnant or breastfeeding * current mean arterial pressure (map) \<60 mmhg while on 2 or more vasopressors at above doses for more than 2 hours * history of any significant cardiac (myocardial infarction within 12 months of screening visit or unstable angina), chronic ongoing hepatic, or renal disease (grade 3 or higher); diagnosis of congestive heart failure with hypoxemia primarily due to decompensated heart failure; diagnosis of severe chronic obstructive pulmonary disease (copd) or interstitial lung disease requiring supplemental oxygen at home * concurrent diagnosis of diffuse alveolar hemorrhage * requiring continuous dialysis (unable to stop dialysis during study agent infusion)

- ventilator support of fio2 >0·8 or peep >20 cm h2o and ongoing use of more than two vasopressors for 2 or more hours with any agent at doses shown below in the supine position. - norepinephrine >12 μg/min or 0.2 μg/kg per min - phenylephrine >150 μg/min or 3 μg/kg per min - epinephrine >10 ug/min or 0.2 μg/kg per min - vasopressin >0.04 units/min - concurrent use of other investigational agents specifically for treatment of ards or inflammatory cytokines. (note: agents established to be efficacious and/or those used outside of formal trials are permitted as supportive data emerge) - known ineligibility for use of a ventilator for a minimum of 7 days, as judged by the institution's triage team - known allergy to msc components: fetal calf serum, human albumin or dmso - active invasive malignant disease requiring chemotherapy/radiation - other concurrent life-threatening disease (life expectancy <6 months) or eligible for hospice care - known history of hiv infection on active treatment - females who are pregnant or breastfeeding - current mean arterial pressure (map) <60 mmhg while on 2 or more vasopressors at above doses for more than 2 hours - history of any significant cardiac (myocardial infarction within 12 months of screening visit or unstable angina), chronic ongoing hepatic, or renal disease (grade 3 or higher); diagnosis of congestive heart failure with hypoxemia primarily due to decompensated heart failure; diagnosis of severe chronic obstructive pulmonary disease (copd) or interstitial lung disease requiring supplemental oxygen at home - concurrent diagnosis of diffuse alveolar hemorrhage - requiring continuous dialysis (unable to stop dialysis during study agent infusion)

- ventilator support of fio2 >0·8 or peep >20 cm h2o and ongoing use of more than two vasopressors for 2 or more hours with any agent at doses shown below in the supine position. - norepinephrine >12 μg/min or 0.2 μg/kg per min - phenylephrine >150 μg/min or 3 μg/kg per min - epinephrine >10 ug/min or 0.2 μg/kg per min - vasopressin >0.04 units/min - concurrent use of other investigational agents specifically for treatment of ards or inflammatory cytokines. (note: agents established to be efficacious and/or those used outside of formal trials are permitted as supportive data emerge) - known ineligibility for use of a ventilator for a minimum of 7 days, as judged by the institution's triage team - known allergy to msc components: fetal calf serum, human albumin or dmso - active invasive malignant disease requiring chemotherapy/radiation - other concurrent life-threatening disease (life expectancy <6 months) or eligible for hospice care - known history of hiv infection on active treatment - females who are pregnant or breastfeeding - current mean arterial pressure (map) <60 mmhg while on 2 or more vasopressors at above doses for more than 2 hours - history of any significant cardiac (myocardial infarction within 12 months of screening visit or unstable angina), chronic ongoing hepatic, or renal disease (grade 3 or higher); diagnosis of congestive heart failure with hypoxemia primarily due to decompensated heart failure; diagnosis of severe chronic obstructive pulmonary disease (copd) or interstitial lung disease requiring supplemental oxygen at home - concurrent diagnosis of diffuse alveolar hemorrhage - requiring continuous dialysis (unable to stop dialysis during study agent infusion)