Patients with severe thermal injuries often display an impaired cell-mediated immune response. Clinically, this is expressd as an increased susceptibility to infection and, occasionally, as a decreased ability to reject unmatched allograft skin. Experimentally, patient serum is suppressive to the PHA response of normal human lymphocytes, which we have found to be mediated through B cell participation in the generation of suppressor T cells. We herein document the ability of bacterial endotoxin to produce these effects, both in vivo and in vitro.