Renal cell carcinoma antigen is a rather specific marker for normal and neoplastic renal tissue. We investigated the expression of this antigen in 34 gonadal and extragonadal germ cell tumors, including 8 pure yolk sac carcinomas and 26 embryonal carcinomas, 15 of which were combined with teratomas, seminomas, and dysgerminomas. Renal cell carcinoma antigen was demonstrated in all 8 yolk sac tumors and 21 of 26 embryonal carcinomas (81%). In yolk sac tumors, renal cell carcinoma antigen reactivity was diffusely present throughout the tumors. In embryonal carcinomas, this marker was identified only in yolk sac components. Both intracytoplasmic and membranous staining patterns were present. No reactivity was noticed in embryonal carcinoma cells, seminoma, dysgerminoma, and other components of teratomas. The study suggests an antigenic similarity between renal tubules and yolk sac tumors. Furthermore, the renal cell carcinoma antigen may be used as an addition to the panel of immunocytochemical markers for yolk sac carcinomas.