ABSTRACT Gonadal development, germ cell differentiation and the appearance of membrane antigenic determinants, specific for male and female germ cells during gonadogenesis, was studied in larval and juvenile carp (Cyprinus carpio L.) until 25 weeks after fertilization. Indirect immunofluorescence studies with four monoclonal antibodies raised against carp spermatozoa revealed that monoclonal antibody WCS 29 stained the outer membranes of primordial germ cells in larvae from 3 days after fertilization. The monoclonal antibodies WCS 3 and 17 reacted with the outer membranes of germ cells from 7 weeks after fertilization onwards, simultaneously with the onset of germ cell proliferation. With monoclonal antibody WCS 28 germ cell membranes were clearly stained from 18 weeks after fertilization. Similar reactions were observed in both sexes, however, female germ cells reacted at an earlier developmental stage with the monoclonal antibody WCS 28 than male germ cells. In the developing testis the monoclonal antibodies stained all types of spermatogenic cells. In the ovary, however, only oogonia and early prophase oocytes showed a positive reaction with the four monoclonal antibodies. The results indicate that germline-specific antigens are present on the outer membranes of primordial germ cells and their male and female descendants, with the exception of elderly oogenic stages. It is assumed that the appearance and disappearance of these membrane antigens reflect differentiation steps of germ cells during gonadogenesis.