Leukotrienes B4, C4, and D4 were capable of replacing the helper cell or interleukin 2 requirement for gamma-interferon (IFN gamma) production by Lyt-1-,2+ cells from C57BL/6 mouse spleen cells at leukotriene concentrations as low as 0.002 microM. An antioxidant inhibitor (butylated hydroxyanisole) of lipoxygenase metabolism of arachidonic acid suppressed IFN gamma production. The suppression was significantly reversed by leukotriene C4, which further suggests that leukotrienes and possibly other substances produced by the lipoxygenase pathway of arachidonic acid metabolism play an important role in the regulation of IFN gamma production. All of these events may be related to activation of guanylate cyclase activity, since cyclic GMP also significantly reversed the suppressor effects of butylated hydroxyanisole in IFN gamma production. The leukotriene help for IFN gamma production was independent of DNA synthesis or cellular proliferation. The data are consistent with the hypothesis that lipxoygenase products of arachidonic acid metabolism may play a role in the mediation of interleukin 2 help in IFN gamma production. Cells that are rich sources of leukotrienes, then, should play important roles in positive regulation of lymphokine production.