Zinc is a mitogen for peripheral blood mononuclear cells (PBMC). The optimal mitogenic concentration was found to be 0.05 mmol/l (327 micrograms/dl), four times higher than physiological serum levels. Maximal proliferation was observed after 6 days. Limited dilution technique revealed a frequency of zinc reactive cells of 1:3467 (median; range 1:1628-1:6235). Cord blood mononuclear cells from four of six healthy children could be stimulated to proliferate by zinc. A normal zinc-induced proliferative response could be demonstrated in all six HIV-infected patients in the Walter-Reed-stage I, in nine of 11 patients in Walter-Reed II and in only two of five patients in Walter-Reed III. In Walter-Reed IV to VI all eight patients showed a weak response to zinc (less than 50% of the healthy day control). Decreased zinc serum levels were found in 10 of 28 patients and in one of 16 controls. There was a significant correlation of a diminished zinc-induced proliferation with lower serum levels of zinc and a reduced proportion of CD4 helper cells in HIV-1-infected men. Because of a suppression of mitogenesis by high dose of zinc an excessive intake of zinc as used by some HIV-1-infected patients can presently not be recommended. The value of zinc-induced proliferation for monitoring HIV-infected patients has still to be established.