Bacterial infection often occurs after trauma and hemorrhage and is believed to be a reflection of a compromised host defense system. In the present study, the itilect of hemorrhage on phytohemagglutinin-induced lymphocyte proliferation was investigated. Lymphocytes obtained from rats 2 h after blood withdrawal in m amount equivalent to 30% of total blood volume plowed a 48% reduction in proliferative response as compared to cells obtained from the same animal before Heeding. This depression in lymphocyte proliferative, capacity appeared to be due to a serum factor or factors duced by hemorrhage. The hemorrhage-induced serum factor(s) is heat-stable, dialyzable, and has an apparent molecular weight between 13,000 and 23,000 M gel filtration chromatography. The hemorrhage-in-4aced factor seems to suppress lymphocyte proliferation in a rapid and irreversible manner. This abnormality in host defense mechanisms may contribute to the Increased incidence of sepsis present after trauma and hemorrhage.