Endotoxin protein, an outer membrane protein of Gram-negative bacteria associated with lipopolysaccharide endotoxin, has been found to be a potent activator of lymphocytes. In the absence of T lymphocytes and macrophages, endotoxin protein can stimulate murine B lymphocytes to synthesize DNA and produce antibodies of diverse specificities. This stimulation is greater than that obtained with two well known murine B cell activators, lipopolysaccharide endotoxin or PPD-tuberculin. Splenic lymphocytes from other species, such as rats, rabbits, and guinea pigs also proliferate when cultured with endotoxin protein. Of particular significance is the finding that endotoxin protein is an activator of human peripheral blood lymphocytes.