Embigin is a member of the immunoglobulin superfamily found in embryonal carcinoma cells. The present study deals with embigin gene expression in adult and embryonic cells. By RNA blot analyses, high levels of embigin mRNA were detected in embryonal carcinoma cells with different differentiation potentials, namely F9 and PCC4 cells. Similar or slightly higher levels of the RNA were detected in the embryonic and extraembryonic portions of 9-day mouse embryos. When F9 cells were induced to differentiate by treatment with retinoic acid and dibutyryl cyclic AMP for 2-5 days, the RNA level increased significantly. On the other hand, embigin mRNA dramatically dropped to almost background levels in embryos, from 11 days postcoitus (p.c.). Many organs of adult mice expressed only low levels of embigin mRNA, but considerable amounts of the transcript were found in the ovary and also in the uterus of pregnant mice (4 days p.c. and 12 days p.c.). In situ hybridization experiments revealed large amounts of embigin RNA in the embryo from 7 to 9 days p.c. The visceral endoderm of 7-day embryos, and the brain, visceral yolk sac and presumptive foregut of 9-day embryos were the sites where intense signals of embigin RNA were localized.