In order to clarify the mechanism of the increased osmotic resistance of red blood cells in lead workers 19 men employed in a lead refining factory and 18 control male workers employed in railway construction were examined for red cell count, haematocrit, MCV, blood and urine lead concentrations, urine coproporphyrin and delta-aminolevulinic acid, osmotic resistance of red cells, lecithin-cholesterol acyltransferase (LCAT) activity in serum, and cholesterol content and cholesterol-to-phospholipid ratio of the red cell membrane. The results were: (1) The osmotic resistance of the red cells (p less than 0.05), cholesterol content of the red cell membrane, blood and urine lead, urine coproporphyrin, and urine delta-ALA concentrations (p less than 0.01) were higher in the lead workers than in the controls. (2) In the lead workers close relationships were observed between the osmotic resistance and the blood lead concentration (r = -0.515, p less than 0.05), osmotic resistance and LCAT activity (r = 0.596, p less than 0.01), and osmotic resistance and cholesterol of the red cell membrane (r = -0.492, p less than 0.05).