A high molecular component of a saline extract derived from Listeria monocytogenes contained amino acids, carbohydrates, and phosphorus. The same fraction was capable of promoting both the in vitro mitogenic and adjuvant activities and the in vivo immunosuppressive activity displayed by the crude extract. The material was mitogenic to B but not to T lymphocytes in vitro. Responses to sheep and horse erythrocytes as well as to lipopolysaccharide were suppressed. Immunosuppression was dose dependent and was present at 1, 2, or 3 days but absent 7 days after injection. Both primary and secondary responses to sheep erythrocytes were impaired.