A quantitative assay of in vitro cell-mediated cytotoxicity has been used to detect the appearance of cytotoxic cells in the spleens of immunized BALB/c mice. Biometric techniques which utilize dose response curves suggested quantitative and qualitative changes in immune activities of different lymphoid populations. The immune responses to intraperitoneal injections of allogeneic tumor cells were studied as functions of both time and immunizing cell dose. Spleen cell-mediated cytotoxicity was detectable 3 days after immunization. The response was maximal after about 10 days and then fell gradually. The highest response to a single intraperitoneal exposure occurred after injection of 10 7 EL-4 cells. Higher doses of cells resulted in lower responses. The response to a second immunizing dose of EL-4 cells was quicker and reached a peak at about 7 days. In addition to changes in total immune activity, a qualitative change occurred in cell-mediated immunity during the course of primary and secondary responses. With an increase in total immune activity, cytotoxic activity of the average immune cell appeared to increase. The time course and immunizing dose dependence of cytotoxic antibody levels and cell-mediated responses were similar after primary immunization. Hyperimmunization resulted in increasing cytotoxic antibody titers but decreasing cell-mediated activity. Cellular immunity in the hyperimmune animals appeared to have been stimulated but effectively suppressed. This suppression was reversible by transfer of the hyperimmune spleen cells with antigen to sublethally irradiated hosts.