Summary The suppressive effect of antilymphocyte serum (ALS) on the immune response to sheep erythrocytes has been studied in complement deficient mice which lack C5. ALS proved to be equally as potent in depressing splenic plaqueforming cells and serum hemagglutinin and hemolysin titers in deficient as compared to normal mice. These findings indicate that antibody mediated, complement dependent immune cytolysis of lymphocytes is not essential for ALS activity since C5 is required for this. They support the view that ALS may contain antibodies which facilitate the elimination of lymphocytes by phagoeytosis (23), rather than by immune cytolysis.