The interactions of calcium and lead on the human erythrocytes have been studied in vitro using 45Ca and 203Pb as tracers. The chemical groups binding calcium and lead on the erythrocytes were also investigated. Calcium ions in the plasma were shown to be capable of replacing the 203Pb on the red cells. More than 85% of the 203Pb in the erythrocyte was associated with the cytoplasmic components, and the rest was bound to the stromal membrane. About 90% of 45Ca was attached to erythrocyte membrane. Extraction of 45Ca and 203Pb-labelled erythrocyte membranes with chloroform/methanol mixture showed that the distribution patterns of these two nuclides are similar, with over 88% protein bound, less than 10% lipid bound, and traces in the aqueous phase. Chemical modification of erythrocyte membrane proteins with carbodi-imide, p-chloromercuribenzoate (PCMB), and maleic anhydride suggested that the carboxyl groups are responsible for binding lead and calcium to the red cell membrane. The SH groups may have a minor role in the binding for both cations. Amino groups did not appear to affect the binding of these cations. Gel chromatography of 45Ca-labelled erythrocyte membrane indicated that Ca++ bound to the same fraction of membrane proteins as 203Pb, corresponding to a molecular weight of about 130 000 to 230 000. A possible implication of these findings is that lead and calcium may compete for the same binding site(s) on the erythrocyte.