Metallothioneins (MT) are ubiquitous, low-molecular-weight proteins that exhibit high binding affinities for heavy metal ions. The expression of these cysteine-rich proteins is induced in response to various types of chemical and physical stresses and therefore can be used to assess human exposure to cytotoxic environmental agents. In the current study, MT levels of human peripheral blood lymphocytes were determined using an MT-specific antibody and flow cytometry. Treatment of human whole blood ex vivo with CdCl2 was found to induce a concentration- and time-dependent increase in lymphocyte MT levels at concentrations as low as 0.3 mu M and within a 12-h period. Interestingly, differences were observed in the magnitude of cadmium-induced MT levels in the lymphocytes of six human test subjects. Two members of the study population exhibited CdCl2-induced cellular MT levels that were up to twofold greater than the lymphocytes of other human subjects. While the lymphocytes of most test subjects exhibited a symmetric (unimodal) distribution of cadmium-induced MT-specific fluorescence, the cells of two individuals displayed a heterogeneous (nonuniform) distribution of MT levels. Dual-parameter flow cytometric analysis using phenotype-specific antibodies indicated that variations in the responsiveness of subpopulations of lymphocytes to CdCl2 were responsible for the heterogeneous distribution of MT-specific cellular fluorescence. Thelper (CD4-positive) and T-suppressor/cytotoxic (CD8-positive) lymphocytes expressed higher cellular levels of MT than other lymphocyte subpopulations (i.e., B lymphocytes, natural killer cells). Our results suggest that MT protein levels of peripheral blood lymphocytes, as determined by this flow cytometric method, may be used to assess human exposure to toxic metals and to characterize various quantitative/qualitative aspects of the response of individuals to cadmium and possibly to other types of environmental stresses.