Human peripheral blood monocytes were cultivated for 1-30 d before assay for H2O2 release or challenge with Leishmania donovani promastigotes (LDP) or amastigotes (LDA). 1-d cells readily generated H2O2 in response to both phorbol myristate acetate triggering (1,013 +/- 58 nmol/mg protein . 90 min) and LDP ingestion, and killed 50% of LDP within 6 h, and 90% by 24 h. In contrast, the same cells released little H2O2 during LDA ingestion, killed no LDA at 6 h and less than 30% by 24 h, and supported intracellular LDA replication. Monocyte-derived macrophages (cells first cultivated for greater than or equal to 7 d) generated less than 125 nmol H2O2/mg . 90 min after phorbol myristate acetate triggering, killed neither LDP nor LDA, and permitted both forms to replicate. The addition of mitogen- or antigen-stimulated lymphokines, however, prevented the decline in monocyte oxidative capacity, enhanced macrophage H2O2 release by more than sixfold, and, in parallel, induced 1-d monocytes to kill LDA and cultivated macrophages to display both promastigocidal and amastigocidal activity. In comparison to 1-d monocytes and lymphokine-activated macrophages from normal donors, the same cells from patients with chronic granulomatous disease (CGD) or normal cells whose oxidative activity had been impaired by catalase pretreatment or glucose deprivation exerted considerably less or no antileishmanial activity during the early (6-24 h) postphagocytic period. By 48 h after infection, however, 1-d CGD monocytes and oxidatively impaired normal cells killed 40 and greater than 80% of LDP, respectively. Although a longer period of lymphokine stimulation was required and the resulting antileishmanial effects were not as rapid as with normal cells, activated CGD monocytes and macrophages also eventually achieved promastigocidal and amastigostatic activity. These results indicate that human mononuclear phagocytes utilize both oxygen-dependent and -independent mechanisms to achieve activity against ingested Leishmania, and also demonstrate (a) the differential susceptibilities of the two forms of L. donovani to intracellular killing, (b) the key role of oxygen intermediates in effective mononuclear phagocyte antimicrobial activity, (c) the capacity of lymphocyte products to enhance oxygen-dependent as well as -independent pathways, and (d) the vulnerability of the monocyte-derived macrophage to Leishmania infection in the absence of lymphokine stimulation.