A two-phase model of allograft immunity was studied. In the first phase, specific immune T-cells were generated by incubation of responder cells with mitomycin-C treated allogeneic stimulator cells of the same H-2 type as mouse mastocytoma tumor cells. In the second step, the ability of the sensitized cells to kill Cr51-labelled P-815 mastocytoma cells was assayed. Alpha-fetoprotein (AFP) was shown to inhibit the generation of immune cytotoxic T-cells at low concentrations (1-100 ng/ml) when added at the beginning of the first phase. When added at the end of the first phase or in the second (killing) phase, AFP was found to have no significant effect on cytotoxicity, indicating that it did not inhibit the killer T-cell once it was generated.