In order to elucidate effects of nickel on human lymphocytes in vitro, peripheral blood mononuclear cells from normal donors were initially tested for viability in the presence of increasing concentrations of two selected nickel salts, sparingly-soluble nickel subsulfide (Ni3S2), and promptly-soluble nickel sulfate (NiSO4). After establishing the toxicity profile, the cells were cultured for 24 h with each compound at three nontoxic concentrations, 0.01 mM, 0.02 mM, and 0.04 mM, to determine its effect on lymphocyte immunophenotype and function. Cells were also cultured in the presence of 0.01-0.04 mM magnesium acetate, Mg(CH3COO)2, while still other cell samples were subjected to a mixture of Mg(CH3COO)2 plus either Ni3S2 or NiSO4 at equimolar concentration. Following the culture, the immunophenotype of the cells was determined by indirect immunofluorescence, using monoclonal antibodies to major differentiation antigens of peripheral blood mononuclear cells, and their natural killer activity toward K562 target cells was measured. Both nickel salts were found to exert distinct effects on lymphocyte phenotype. Exposure of cells to Ni3S2 resulted in the decline of CD4 and natural killer cell populations. NiSO4 diminished the abundance of natural killer cells and, to a limited extent, also of CD4 cells. The nickel salts tested suppressed natural cytotoxicity of peripheral blood mononuclear cells, with Ni3S2 acting more strongly than NiSO4. The addition of Mg(CH3COO)2 to a nickel salt during in vitro culture abolished the above inhibitory effects. Nickel and magnesium salts did not affect CD3, CD8, CD20, and CD11a cell populations. The results indicate that nickel salts have deleterious effects on human peripheral blood mononuclear cells in short-term in vitro culture, but the magnitude of these effects varies, depending on the cell subsets.