During normal embryonic development, mammalian germ cells use both cell migration and aggregation to form the primitive sex cords. Germ cells must be able to interact with their environment and each other to accomplish this; however, the molecular basis of early germ cell adhesion is not well characterized. Differential adhesion is also thought to occur in the adult seminiferous tubules, since germ cells move from the periphery to the lumen as they differentiate. In a screen for additional adhesion molecules expressed by the germ line, expression of the homophilic adhesion molecule, Ep-CAM, was identified in embryonic, neonatal and adult germ cells using immunocytochemistry and flow cytometry with an Ep-CAM-specific monoclonal antibody. At embryonic stages, germ cells were found to express Ep-CAM during migration at embryonic day 10.5 and early gonad assembly at embryonic day 12.5. Expression of Ep-CAM was also found on neonatal male and female germ cells. In the adult testis, Ep-CAM was detected only on spermatogonia, and was absent from more differentiated cells. Finally, embryonic stem cells were shown to express this receptor. It is proposed that Ep-CAM plays a role in the development of the germ line and the behaviour of totipotent cells.