This paper continues a comparative study (A. D. Nickol and P. F. Bonventre, Infect. Immun. 50:160-168, 1985) describing immune responses exhibited by congenic, Lshs mouse strains C57B1/10 (cure) and B10.D2 (noncure) during the course of disseminated leishmaniasis. We report that sublethal whole-body irradiation of B10.D2 mice before challenge with Leishmania donovani converted the noncuring mice to a curing phenotype. Splenic lymphocytes from L. donovani-infected B10.D2 mice failed to proliferate in response to parasite antigen stimulation in vitro. Splenic lymphocytes from irradiated, cured B10.D2 mice regained the capacity to respond to the parasite antigen stimulus. Transfer of T cells but not B cells from L. donovani-infected B10.D2 mice prevented the acquisition of immunity and recovery from infection in X-irradiated mice. In addition, a splenic T-cell population from L. donovani-infected B10.D2 mice suppressed the proliferation in vitro of parasite antigen-stimulated lymphocytes of irradiation-cured B10.D2 mice. Suppressor T cells were not demonstrable in the spleens of spontaneously cured C57B1/10 mice. Splenic lymphocytes from infected B10.D2 mice were deficient in the production of macrophage-activating factor (MAF) upon stimulation by L. donovani antigens in vitro. Deficient MAF production was specific for parasite antigen stimulation, because MAF production subsequent to concanavalin A stimulation of splenic lymphocytes from infected B10.D2 mice was not suppressed. The data suggest that a genetically based immunological defect in B10.D2 mice prevents the acquisition of effective cell-mediated immunity and subsequent elimination of L. donovani from tissue macrophages. The immunological deficit, not apparent in the curing C57B1/10, appears to be caused by the development of parasite antigen-specific suppressor T cells during the course of the disseminated leishmaniasis.