Two macroscopically distinguishable components (designated soft and firm) of a human testicular germ cell tumor with borderline histology between seminoma, embryonal carcinoma and yolk sac tumor, were maintained as xenografts in nude mice for over 20 passages. Levels of beta-human chorionic gonadotropin (beta-HCG) and of alpha-fetoprotein (AFP) were normal in the patient's serum and were undetectable by immunohistochemical studies of the surgical specimen. The xenografted soft part, however, with morphologic characteristics of an embryonal carcinoma, secreted beta-HCG and AFP during early passages. The firm variant, histologically resembling a seminoma, did not produce these markers. Chromosomal and flow cytophotometric analyses showed genetic differences between the clonally stable variants. A common origin, however, is indicated by two similar marker chromosomes present in both variants.