Studies were carried out to determine changes in lymphoid organs i.e. spleen, lymph node and bone marrow (BM) in progressive experimental visceral leishmaniasis. Mononuclear phagocytes in the BM were increased; spleens showed a hypercellularity coupled with a rise in parasite burden while secondary follicles with no apparent depletion of paracortex were seen in the lymph node. This enhanced proliferation of mononuclear phagocytes in the BM and probably their subsequent recruitment in the spleen could be induced in naive recipients by injecting nylon wool-purified lymph node cells derived from infected mice together with sonicated leishmanial antigen(s). Similar changes could also be induced in the BM of naive recipients by injecting serum of infected mice. In control experiments where both donor and recipients were uninfected such changes were not apparent. A working hypothesis is proposed to delineate the role of lymphoid organs in the pathogenesis of experimental visceral leishmaniasis.