Serum concentrations of anti-hippocampal antibodies and in vitro production of the lymphokine interleukin-2 (IL-2) in response to phytohaemagglutinin (PHA) stimulation were determined using an enzyme immunoassay in 49 schizophrenic patients and 41 healthy controls. Decrease in IL-2 production, a finding frequently associated with many autoimmune diseases, was associated with an elevation in anti-hippocampal antibody optical density (AHA-OD) in schizophrenic patients. Although some control subjects had elevated antibody levels, this elevation was not associated with decreased IL-2 production. Low IL-2 production is well known to be a state marker associated with active autoimmune disease. We suggest that production of hippocampal antibody is a trait marker of vulnerability to autoimmune diseases. Thus, our finding of low IL-2 production in patients with high concentrations of hippocampal antibody is compatible with the possibility that such patients have an ongoing autoimmune process.