Schistosome-derived inhibitory factor (SDIF) was shown to inhibit lymphocyte proliferation. While SDIF is not toxic to lymphoid cells, the generation of cytotoxic effector cells was inhibited by SDIF in a mixed lymphocyte culture. This inhibitory effect was not attributable to the induction of suppressor cells, as SDIF also inhibited the development of nonspecific suppressor cell activity in 7-day cultures of unstimulated spleen cells. The interleukine 2-dependent proliferation of cytotoxic T lymphocytes of or of blast cells was markedly reduced by the addition of SDIF. In contrast, the production of interleukine 2 itself was not impaired by SDIF. These results support the hypothesis of an inhibition by SDIF of a particular step of the mitotic cycle, probably posterior to the G0-G1 transition. An inhibitory activity of SDIF on the expression of the cytolytic activity of cytotoxic T lymphocytes was also observed.