In vivo, tumor cell killing was monitored with 131I-IUdR-labeled tumor cells and whole-body measurement of retained radioactivity. Treatment with antiserum in quantities that were not sufficient to kill the total leukemia cell inoculum (i.e., antigen excess) caused an immunopotentiation of the active immune response; this was manifested by an accelerated rate of tumor cell killing beginning between days 10 and 11. The results obtained in vivo were confirmed by in vitro quantitation; both the cytotoxic antibody and cell-mediated immune responses were potentiated by the injection of antiserum. Immunosuppression was also observed in passively immunized mice. Whether potentiation or suppression occurred was dependent on the relative amounts of antiserum and leukemic cells injected and the innate immunogenicity (and/or antigenicity) of the leukemic cells; antibody excess and high immunogenicity favored suppression.