We have previously demonstrated that histamine can inhibit human helper T cells by direct interaction with these cells. It has now been investigated whether histamine inhibits lymphokine production by various subsets of CD4+ human T cells separated with the Leu-8 (p80) and Leu-18 (anti-CD45R;p220) monoclonal antibodies (MoAb). Histamine was shown to suppress to a similar extent the production of interleukin 2 (IL-2) and gamma interferon (IFN-gamma) by Leu 3+, Leu 3+8+, and Leu 3+8- cell subsets. Mitogen-activated, unseparated Leu 3+ and purified Leu 3+8- cells produced maximal amounts of IL-2 after 24 h and IFN-gamma after 72 h of culture. In contrast, the Leu 3+18+ subset produced no IL-2 after 24 h, and maximal amounts of IL-2 no sooner than 48 h of culture, and only small amounts of IFN-gamma during the entire culture period of 96 h. Histamine suppressed the production of IL-2 by both subsets, both when produced early (after 24 h), as in the case of the Leu 3+18- subset, and late (after 48 h of culture), as for the Leu 3+18+ subset. The IFN-gamma production by the Leu 3+ and Leu 3+18- cells and the marginal production by Leu 3+18+ cells were significantly suppressed by histamine. Dual staining with Leu 8 and Leu 18 MoAb demonstrated that the Leu 18- cell compartment included both Leu 8+ and Leu 8- cells. It was shown that the inhibitory effect of histamine on the early production of IL-2 and the major production of IFN-gamma by T helper cells is mediated via action on both the Leu 3+18-8- and the Leu 3+18-8+ cells. The inhibitory effect of histamine on the late production of IL-2 is mediated mainly via action on Leu-18+ cells.