The efficiency of immunotherapy with murine recombinant interferon-gamma (rIFN-gamma) in mouse visceral leishmaniasis caused by Leishmania donovani was examined. To avoid the side effects encountered after the in vivo administration of high dosages of free IFN-gamma, this lymphokine and muramyltripeptide (MTP-PE) were encapsulated into multilamellar liposomes. We established that a combination of 5 X 10(3) U of IFN-gamma and 6 micrograms of MTP-PE, encapsulated in liposomes and given i.v. in C56BL/6 and BALB/c mice activates macrophages from spleen and liver in vivo to kill L. donovani in vitro. Neither empty liposomes nor the same concentration of free IFN-gamma and/or MTP-PE injected i.v. resulted in a leishmanicidal activity of these macrophage populations. For verification of these results in an in vivo infection model, susceptible mice were infected with L. donovani and were treated with IFN-gamma and MTP-PE encapsulated in multilamellar vesicles. Treatment consisted of multiple i.v. injections beginning 4 and 2 days before infection (prophylactic), either simultaneously with the infection or at various times of the exacerbation and remission phases of visceral leishmaniasis. These mouse strains treated with IFN-gamma and MTP-PE in liposomes had significantly fewer splenic parasites than untreated mice or control animals treated with free drugs or empty liposomes. The targetting of multilamellar vesicles to liver and spleen make them particularly suited for the delivery of macrophage-activating substances used for treatment of visceral L. donovani infection.