Total population and T cell enriched fractions of rat spleen were cultivated in diffusion chambers implanted intraperitoneally to mice and rats. 10-16% of the input cells were recovered after 5 days. When the chambers were carried in the xenogeneic environment activation occurred as indicated by blastogenesis and non discriminative cytotoxicity. Specific activation of the T population was induced by mixed lymphocyte tumor culture in chambers implanted to rats. The presence of tumor cells induced blastogenesis, elevation of the proportion of Fc receptor positive cells and generated cytotoxic cells to the sensitizer tumor. The precursor of cytotoxic cells did not have Fc or C3 receptors since nylon wool column passed fractions depleted from these cells by elimination of EA or EAC rosettes were also activated.