The role of TNF-alpha in visceral leishmaniasis is ambivalent, the eventual outcome of this infection, cure or generalization, being determined by the relative amounts of cytokines produced in vivo. Since release, by monocytes/macrophages, of TNF-alpha and interleukins 1 beta (IL-1 beta) and IL-6 is important in both the induction and effector phases of the immune responses, these mediators were determined in sera and cell culture supernatants of seventeen L. donovani infected patients in Brazil. The results are compared to those of a local control group. Circulating immunoreactive TNF-alpha in patients (median, 140 pg ml-1) was increased ten-fold over controls (median 16 pg ml-1, p less than or equal to 0.0001). In contrast, serum IL-1 beta was less than 20 pg ml-1 in all patients, although detectable in sera of 3/16 Brazilian controls (chi 2 = 3.5, p less than 0.1). Mitogen induced in vitro release of IL-1 beta and IL-6 by patients' circulating mononuclear cells was significantly reduced, and the capacity of patients' peripheral monocytes for H2O2 generation in response to opsonized zymosan was significantly diminished. In the patients, serum TNF-alpha levels were inversely related to IL-1 beta release in vitro (rho = -0.57, p less than or equal to 0.01).