Leukotriene B4 (LTB4) can regulate several lymphocyte functions, including the augmentation of cytotoxic activity and the induction of suppressor cells. When T lymphocytes were preincubated with picomolar concentrations of LTB4, they would suppress the proliferative response of unfractionated peripheral blood mononuclear leukocytes to concanavalin A in a subsequent co-culture system. Such a suppression did not occur when the responding population was depleted of monocytes. Furthermore, the effect was reversed to an enhancement when the responding unfractionated population was treated with indomethacin, suggesting a role for monocytes and cyclooxygenase products in the effector phase of LTB4-induced suppressor activity. When sorted into T4+ and T8+ cells before preincubation with LTB4, both T cell subsets could be induced by LTB4 to exert suppression. T4+ cells, however, required the presence of monocytes in the responder population in order to manifest suppressor activity, whereas T8+ cells were active even in the absence of monocytes. When LTB4-preincubated T cells or T4+ cells were sorted into T4+ and T8+ subsets after preincubation, suppressor cell activity was found only in the T8+ subset. Furthermore, T8+ cell-depleted T lymphocyte cultures, incubated for 24 hr with LTB4, showed a significant increase in the proportion of T8+ cells. Together, these data suggest that LTB4 induces suppressor T cells which can derive from either T4+ or T8+ subpopulations but which are phenotypically T8+ when exerting their suppressive activity. Thus, by interacting with both T4+ and T8+ lymphocytes, LTB4 can modulate immune responses with the cooperation of functionally competent accessory monocytes.