Objective: Although mesenchymal stem/stromal cells represent a promising therapeutic strategy for acute respiratory distress syndrome, clinical translation faces challenges, including scarcity of bone marrow donors, and reliance on bovine serum during mesenchymal stem/stromal cell proliferation. We wished to compare mesenchymal stem/stromal cells from human umbilical cord, grown in xeno-free conditions, with mesenchymal stem/stromal cells from human bone marrow, in a rat model of Escherichia coli pneumonia. In addition, we wished to determine the potential for umbilical cord-mesenchymal stem/stromal cells to reduce E. coli-induced oxidant injury.Design: Randomized animal study.Setting: University research laboratory.Subjects: Male Sprague-Dawley rats.Interventions: Acute respiratory distress syndrome was induced in rats by intratracheal instillation of E. coli (1.5-2 x 10(9) CFU/kg). "Series 1" compared the effects of freshly thawed cryopreserved umbilical cord-mesenchymal stem/stromal cells with bone marrow-mesenchymal stem/stromal cells on physiologic indices of lung injury, cellular infiltration, and E. coli colony counts in bronchoalveolar lavage. "Series 2" examined the effects of cryopreserved umbilical cord-mesenchymal stem/stromal cells on survival, as well as measures of injury, inflammation and oxidant stress, including production of reactive oxidative species, reactive oxidative species scavenging by superoxide dismutase-1 and superoxide dismutase-2.Measurements and Main Results: In "Series 1," animals subjected to E. coli pneumonia who received umbilical cord-mesenchymal stem/stromal cells had improvements in oxygenation, respiratory static compliance, and wet-to-dry ratios comparable to bone marrow-mesenchymal stem/stromal cell treatment. E. coli colony-forming units in bronchoalveolar lavage were reduced in both cell therapy groups, despite a reduction in bronchoalveolar lavage neutrophils. In series 2, umbilical cord-mesenchymal stem/stromal cells enhanced animal survival and decreased alveolar protein and proinflammatory cytokine concentrations, whereas increasing interleukin-10 concentrations. Umbilical cord-mesenchymal stem/stromal cell therapy decreased nicotinamide adenine dinucleotide phosphate-oxidase 2 and inducible nitric oxide synthase and enhanced lung concentrations of superoxide dismutase-2, thereby reducing lung tissue reactive oxidative species concentrations.Conclusions: Our results demonstrate that freshly thawed cryopreserved xeno-free human umbilical cord-mesenchymal stem/stromal cells reduce the severity of rodent E. coli-induced acute respiratory distress syndrome. Umbilical cord-mesenchymal stem/stromal cells, therefore, represent an attractive option for future clinical trials in acute respiratory distress syndrome.