SummaryGolden hamsters (Mesocricetus auratus) infected withLeishmania donovanidevelop a disease similar to human kala‐azar. There is conspicuous hypergammaglobulinaemia and their T cells do not respond to stimulation by parasite antigens. The impairment of the cellular immune response seems to be restricted to parasite antigens since infected animals are able to develop a T cell response to the mitogen Concanavalin A (Con‐A) and, after sensitization, to the antigens keyhole limpet haemocyanin (KLH) and human serum albumin (DNP‐HSA). In the present investigations we studied the role played by infected macrophages in the development of the cellular unresponsiveness present in visceral leishmaniasis. Adherent spleen cells from infected hamsters were unable to presentL. donovaniantigens to antigen specific T cells, however they were able to present KLH. Conversely. T cells from infected animals did not respond to parasite antigens even when these antigens were presented by normal syngeneic macrophages. Interestingly, lymphocytes from inguinal lymph nodes of infected animals sensitized in their foot pad with parasite antigens proliferated well when stimulated in vitro withL. donovaniantigens. These results suggest that the defect in the cellular immune response of theL. donovaniinfected hamsters is a consequence of a selective inability of their antigen presenting cells to process and present parasite antigens to T