Endotoxin protein (EP) has been shown to be a mitogen and polyclonal activator of human peripheral blood lymphocytes. EP stimulates proliferation of B lymphoyctes in the absence of T cells, and this activation is nonspecific by a number of parameters. Additionally, EP mitogenesis, but not polyclonal activation, is inhibited in the presence of human serum, suggesting that these events are dissociable. In these studies, EP appears to be equivalent to or better than pokeweed mitogen in stimulating nonspecific antibody production in vitro.