Factors obtained from phorbol myristate acetate (PMA)-stimulated EL-4 thymoma cells, a continuous T cell line, suppressed lymphokine-induced macrophage activation to kill intracellular Leishmania tropica amastigotes. Suppression of this macrophage effector activity was dependent upon concentration of EL-4 fluids admixed with lymphokines in infected macrophage cultures, and was not due to residual PMA or factors released from unstimulated EL-4 cells. Fluids from PMA-stimulated EL-4 cells did not affect the expression of microbicidal activity by macrophages activated in vivo as a consequence of infections with Mycobacterium bovis strain BCG, nor did they abrogate intracellular killing activities by C3H/HeJ macrophages primed by BCG infection and triggered by lymphokines in vitro. That the action of this EL-4 suppressor activity was at the priming stage of macrophage activation was confirmed by kinetic studies: EL-4 fluids added to lymphokine-treated cells in the first 4 hr of treatment completely suppressed intracellular killing of L. tropica; fluids added after 4 hr were not effective. The effects of these EL-4 factors appeared to be selective: of three effector activities of activated macrophages tested, induction of resistance to infection, tumor cytotoxicity, and intracellular destruction of L. tropica, only intracellular killing by lymphokine-treated macrophages was significantly suppressed. These T cell-derived soluble suppressor factor(s) may provide insight into mechanisms of immunosuppression during leishmanial disease and perhaps other intracellular parasitic infections.