To clarify the function of Pb-binding protein and the interaction between lead and selenium in human erythrocytes, the following experiments were performed. (a) blood from a man who has not been exposed to lead occupationally, (b) blood from lead-exposed workers and (c) blood from control subjects working in the same plant were used. (a) was incubated with Pb and/or Se. Hemolysates of rinsed erythrocytes were applied onto a Bio-gel A 1.5 m column and eluted by 40 mM Tris-HCl buffer (pH 7.0). As the results, (1) Pb in erythrocytes was found in ALAD fraction. When Pb was added in vitro at 1.25 microM of final concentration, Pb content of ALAD fraction increased and the other Pb-containing fraction appeared at the position of a smaller molecular size than that of Hb fraction. When Pb dose added was increased to 2.5 or 10 microM, Pb contents in the newly appeared fraction was larger than that in ALAD fraction. The affinity of Pb with the protein in the new fraction was weak, and Pb was released from the protein by ultrafiltration. (2) There were main and sub-peaks containing Pb in erythrocytes from Pb-workers; the former was ALAD fraction and the others were smaller molecules than ALAD. The peak of Pb observed with the blood from a worker who has worked at the same plant for more than 20 years (chronic exposure) corresponded with the new peak which appeared in vitro Pb exposure. (3) As to the interaction between Pb and Se, there was no effect of Se added in vitro on the position of Pb-containing fraction and on Pb amounts in the chromatographic profile. When Se was incubated with blood, however, coexistence of Pb made Se concentration in erythrocytes high compared with the case of Se alone.