The immunosuppressive effects of anti-lymphocyte serum (ALS) administered alone, or in conjunction with adult thymectomy, were examined in mice subsequently immunized to one of six different antigens: sheep erythrocytes (SRBC), goose erythrocytes (GRBC), rat erythrocytes (RRBC), keyhole limpet hemocyanin (KLH), horse spleen ferritin (HSF), and polyvinyl pyrrolidone (PVP). The thymus-dependency of the immune responses to most of these antigens was known to vary considerably and the antigens were chosen for this reason. A striking range of differences manifested by ALS alone, or in conjunction with thymectomy, was observed, ranging from almost total suppression of immune responsiveness to some antigens (GRBC and SRBC), to moderate suppression of others (HSF and RRBC), to enhancement (PVP) or both initial 19S antibody enhancement and subsequent 7S antibody suppression (KLH). The results support the view that the immunosuppressive effects of ALS on humoral antibody formation are highly variable and inconsistent, that a major factor causing this variability is the thymus-dependency of antigen, and further, that the concept of thymus-dependency of antigens is entirely a relative one.