The effects of factors secreted by polymorphonuclear leucocytes (PMNL) incubated with multiple sclerosis (MS) and control brain antigens (MSG2 and KG2 respectively) were evaluated by assay of thymidine uptake by lymphocytes from normal individuals and MS patients. It was found that the cellular uptake of thymidine of both MS and normals was significantly lower in the presence of MSG2-PMNL supernatants compared to that in the presence of KG2-PMNL or without antigen-PMNL supernatant. The findings indicate that an inhibitor of lymphocyte proliferation either may be released from PMNL by the action of MSG2 or may be synthesized in response to challenge by MSG2.