During the first 2 to 4 weeks of progressive visceral infection with the intracellular protozoan, Leishmania donovani, spleen cells from BALB/c mice failed in response to leishmanial antigen to produce either of the activating T cell-derived lymphokines, interleukin 2 (IL 2) or gamma-interferon (IFN-gamma). Four weeks after infection, however, antigen-induced IL 2 and IFN-gamma secretion emerged and coincided with the onset of control over parasite replication and the subsequent killing of greater than 80% of intrahepatic L. donovani. The development of this immunosecretory activity correlated with the hepatic tissue response at the site of parasitized Kupffer cells. This response progressed from Kupffer cell fusion (week 1) to fusion plus a mononuclear cell infiltrate (week 2) to well-organized granuloma formation (weeks 4 to 8). In contrast, T cell-deficient nude BALB/c mice exerted no control over L. donovani, their spleen cells failed to generate antigen-induced IFN-gamma, and at 4 weeks, their livers were devoid of any tissue reaction. Since spleen cells from 2-week infected normal mice did not produce antigen-stimulated IL 2 or IFN-gamma, these mice were treated with recombinant (r) lymphokines. Various protocols using both high and low dose human rIL 2 had no antileishmanial effect. Hepatic parasite replication was completely halted, however, by macrophage-activating doses of murine rIFN-gamma. These results reemphasize that an intact T cell-dependent response is required for successful defense against L. donovani, indicate that this immune response can be measured at both the cellular (secretory) and tissue levels, and confirm that IFN-gamma can exert an antileishmanial effect in vivo.