We have investigated the relative importance of two lymphokines, IL-2 and IFN-γ, in the primary murine MLR. Three separate lines of evidence indicate that IL-2 and not IFN-γ is the relevant lymphokine for both proliferation and activation of CTL in these cultures. No obligate role for IFN-γ was found. First, CTL activation in the presence of high dose cyclosporin A was partially reconstituted with IL-2, although no detectable IFN-γ was produced in such cultures. In addition, IFN-γ could not reconstitute cyclosporin-inhibited cultures. Second, inclusion of a high concentration of several distinct anti-IFN-γ antibodies failed to inhibit MLR cultures. Third, CTL activation by stimulator cells inactivated by UV irradiation was reconstituted by IL-2, but not by IFN-γ. These data do not support an autocrine role of IFN-γ in CTL activation and confirm the importance of IL-2 in the primary murine MLR.