The possible relationship among malignancy, differentiation, and chromosomal constitution of primary embryo-derived tumors was studied. Tumors were induced by transplanting 7-day-old mouse embryos under the kidney capsule of syngeneic BALB/c recipients. Transplantation of 101 embryos resulted in 18 tumor-bearing mice: 36 teratocarcinomas; 18 teratomas; and 27 yolk sac tumors. Some of the yolk sac tumors proved to be retransplantable for several generations. Cytogenetic investigation of the primary embryo-derived tumors revealed that the majority of teratocarcinomas (82%) were chromosomally normal, whereas almost all (83%) karyotyped teratomas and yolk sac tumors had a highly abnormal chromosomal constitution. Most common aberrations were polyploidy; overrepresentation of chromosome 1, 6, 15, or 19; and an underrepresentation of chromosome 2, 4, 14, or a sex chromosome.