Sickled erythrocytes served as a marker for maternal blood in a cell transfer study of placentas from uncomplicated term pregnancies of women with sicklemia. The incidence of concurrent sickling in maternal and fetal blood was 100% in the 44 cases examined. Maternofetal passage of the erythrocytes was found four times more frequently in umbilical or chorionic veins (8.4%) than in arteries (1.9%). Deported chorionic villi were found in the veins in three of 44 cases but not in the arteries. These findings indicate fresh maternofetal leakage of blood during the third stage of labor.