Visceral leishmaniasis (VL) and cutaneous leishmaniasis (CL) are the major parasitic diseases in which the immune system is implicated in pathogenesis. The in vitro production of interleukin (IL)-2, IL-4, IL-6, IL-8, tumor necrosis factor-a (TNFa), and granulocyte-macrophage colonystimulating factor (GM-CSF), spontaneously and in response to stimulation of peripheral blood mononuclear cells with anti-CD3, phytohemagglutinin, and lipopolysaccharide, was investigated to determine their importance in VL and CL. Highly enhanced production of IL-4, IL-6, IL-8, and TNFa was seen in VL. However, enhanced production ofIL-4 and TNFa but almost normal production of IL-6 and IL-8 was seen in CL. The highly increased IL-4 production was the most characteristic and common feature of both VL and CL. Of interest, highly deficient GM-CSF production may be implicated in abnormalities in synthesis of hematopoietic lineage of cells in these diseases.