SUMMARY Cellular immunity occurring in Lewis rats that received either Lewis × Brown Norway (L × BN) kidney or BN fibrosarcoma allografts was measured by incubation of their lymphoid cells with 51Cr-labeled BN fibrosarcoma cells. The release of 51Cr from these target cells was the measure of cytotoxicity. The cytotoxic effect of sensitized lymphoid cells was specific. Cytotoxicity was first detected 3–4 days after immunization, reached a peak at 5 days, and declined sharply to low levels that were barely detectable 3 weeks after renal grafting. A similar pattern was present after tumor immunization, except that cytotoxicity declined more rapidly, falling to nondetectable levels by 10–12 days. The peak response at 5 days correlated closely with the rejection of BN tumor and renal allografts. A secondary cellular immune response occurred 1 day earlier than the primary. Cytotoxic and hemagglutinin antibodies appeared concurrently with the cellular immunity. Antibody titers, after reaching a peak at 5–8 days, declined to low levels and then rose again approximately 2 weeks later without further immunization.