To determine peripheral blood lymphocyte and red blood cell glutathione concentration in ARDS patients at study entry and at days 1,3,7,14, and 21. These data will be correlated with clinical parameters of lung injury. To determine the effect of Procysteine therapy on the activation status of circulating poly-morphonuclear neutrophils (PMNs) by measuring hydrogen peroxide generation after ex vivo stimulation and to compare these data with determination of PMN intra-cellular glutathione concentration.