Spleen cells from DBA/2 mice, after immunization with syngeneic P815 mastocytoma cells and Corynebacterium parvum, respond to P815 in vitro with a brisk, secondary-type generation of cytotoxic cells. This cytotoxicity is mediated by antigen-specific T-lymphocytes and correlates with resistance to in vivo challenge. This model confirms the observations of previous investigators made in semisyngeneic hosts using an in vivo transfer model. Spleen cells from "early" tumor-bearing hosts (TBHs), 7-12 days after intradermal (i.d.) inoculation of 10(6) P815 cells alone, made a similar, but generally higher, cytotoxic T-lymphocyte (CTL) response in vitro. Spleen cells from "late" TBHs (18-28 days) completely suppressed the in vitro CTL response of immune cells (e.g., from 71% specific release in controls down to 8% at an effector: target ratio of 40:1). Early i.d. TBH spleen cells, because of their higher level response, appeared to be resistant to this suppression (85% release for controls and 84% when suppressor cells were added at 40:1). By testing early TBH CTL at lower effector: target ratios, however, suppression by late TBH spleen cells could be readily demonstrated. When TBHs were inoculated s.c. instead of i.d. or with lower doses of tumor cells, responses were lower and susceptibility of splenic CTLs to suppression was increased. At intermediate times after tumor inoculation (14-20 days), spleen cells from TBHs still can respond in vitro, but they are completely suppressed by spleen cells from late TBHs. The suppressor cells are antigen-specific, radiation-sensitive, Thy1+ cells.