The effect of purified lipophosphoglycan (LPG) of Leishmania donovani on signal transduction and gene expression in murine bone marrow-derived macrophages was investigated. LPG stimulated the rapid expression of both c-fos and TNF genes within 30 min after exposure, suggesting that the interaction of LPG with its receptor stimulated a specific signal transduction pathway. Macrophages pretreated with LPG for 3 h became unresponsive to subsequent stimulation with LPS and the activators of protein kinase C, 1-oleoyl-2-acetyl-glycerol, and calcium ionophore A23187. Moreover, LPG induced a rapid down-modulation of TNF receptors. In contrast, the ability of macrophages to express the c-fos gene in response to the cAMP analogue, dibutyryl cAMP, was not impaired by LPG. Fragmentation of LPG revealed that the inhibitory activity of LPG required both the repeating phosphorylated disaccharides and the phosphosaccharide core. Collectively, these data demonstrate that LPG selectively impaired signal transduction in macrophages and suggest a role for this molecule in the survival of the parasite within the macrophage.