A microcytotoxicity assay (MCA) and a (51)Cr release cytotoxicity assay (CRA) for cell mediated immunity to the allogeneic tumor were done in parallel with same reagents used for both assays. Therefore, CRA was modified in which monolayer culture cells were used as target cells and MCA was devised in which the adherent surviving target cells after the interaction with effector cells were assessed by (51)Cr uptake (so called (51)Cr post-labeling assay...CPLA). Under standard conditions, CPLA detected a significant activity of weakly sensitized lymphocytes which was not detected by CRA. Reactivity of lymph node cells after the tumor transplantation was tested simultaneously in CRA, CPLA and the macrophage migration inhibition test (MIT). The data showed that CPLA detected immune reactivity for longer period than CRA, and as long as MIT. The lytic activity of lymph node cells 8 days after immunization was monitored by a long term CRA (40-hours' incubation). The cytotoxicity value for late 20 hr was remarkably higher than for early 20 hr. This in vitro activation of sensitized lymphocytes indicated a peak at 8 days and then declined, dissappeared at 14 days. When both assays were done under same condition, CPLA detected a moderate activity which was not detected by CRA. This result means that CPLA could detect immune responses which inhibit tumor cell growth without killing them. Both assays detected primarily a T lymphocyte-mediated activity. The CPLA also detected a weak non T-cell activity in anti-θ plus complement-treated alloimmune lymph node.