Three human testicular teratocarcinomas were serially passaged following subcutaneous transplantation into nude mice. Tumor cell suspensions from selected passages were injected intraperitoneally. The subcutaneous transplants of each tumor conserved the morphological characteristics of one component of the primary tumor, namely an embryonal carcinoma in one case and a yolk sac tumor in two. The latter maintained the capacity to synthesize alpha-fetoprotein (AFP). After intraperitoneal injection of cell suspensions, tumors, either attached to or even invading abdominal organs or in the form of free-floating tumor spheroids, were observed. AFP could be localized within the attached growths but not in the spheroids. A critical tumor volume and/or vascularization seemed to be necessary for AFP production in tumor cells. In spheroids from one tumor, cytogenetic analysis revealed both human and murine cells. Thus, these spheroids, apparently composed of tumor cells in the center and murine cells at the periphery, can not be considered to be embryoid bodies.