Spleen lymphocytes from donor mice immunized against rabies virus were transferred into non immunised cyclophosphamide treated syngeneic mice. Antibody producing cells against rabies antigens were increased in the cyclosphosphamide treated recipient mice (p less than 0.01) as compared to then non treated recipient mice. A fusion was made with the splenic cells of cyclophosphamide treated recipient mice and SP2 murine myeloma cells 30 of 144 hypridoma plated welles were positive. There results suggest that cyclophosphamide immunosuppression inhibits the regulation of antibody production.