The blastogenic response of MS lymphocytes to a group of paramyxovirus antigens and to phytohemagglutinin was investigated. Comparative studies were performed in a selected group of patients before and after transfer factor therapy. There was no blastogenic response of lymphocytes to the paramyxovirus antigens tested in MS patients and normal controls, as measured by the incorporation of 14C thymidine. The stimulation index was below 2.5 even after transfer factor therapy. Lymphocyte transformation induced by PHA appeared significantly depressed in the MS patient group before transfer factor therapy when compared with the normal control group. The presence of serum factors that may depress PHA-induced lymphocyte transformation were looked for and not found. Following transfer factor therapy, an increase of the PHA response was observed in the MS group, such that a statistically significant difference in response compared with the normal control group was no longer present.