Peripheral blood lymphocytes, serum immunoglobulins (IgG, IgA and IgM), C3 and C4 complement protein concentrations of 25 male lead-exposed workers from storage-battery plants were examined and compared to 25 healthy male controls. Lead exposure was assessed using blood lead levels measured by atomic absorption spectrophotometry and zinc protoporphyrin (ZPP) levels assayed by hematofluorometry. The absolute number and the percentage of functionally different subsets of peripheral blood mononuclear lymphocytes, i.e. T, T-suppressor, B and natural killer cells, were unchanged. However, T-helper lymphocytes were significantly lower in lead-exposed workers compared to healthy controls (P < 0.05). In addition, lead-exposed workers had a significant reduction in the IgG, IgM and C3, C4 complement levels (P < 0.05). These results suggest that human chronic exposure to lead may be detrimental to the immune system.