DDD mouse embryos at 12-16 days of gestation have a putative 'fetal'α-globin polypeptide chain, detected by its lower electrophoretic mobility than the authentic α-globin chain on acidic urea-Triton gel. When hepatic erythroid cells of DDD embryos were contaminated with yolk sac erythroid cells, two α-bands were visible, but only 'fetal'α seemed to be synthesized, because in embryos of strain ddY which share a common ancestry with DDD, only 'fetal'α was detectable by autoradiography when the hepatic erythroid cells of embryos of more than 15 days of gestation showed an authentic adult α-globin chain other than this 'fetal'α-globin chain. If this 'fetal'α-globin chain is structurally different from the yolk sac α and adult peripheral blood α-globin chains, then switching over of the transcription of α-globin polypeptide chain occurs in hepatic erythroid cells of mid-late DDD embryos. However, it is still possible that this 'fetal'α-globin chain is formed by post-translational modification of the authentic α-globin chain. The α-globin chains of hepatic erythroid cells of C57BL/6 and BALB/c embryos had similar mobilities to those of yolk sac and adult erythroid cells.