Intact genital ridges containing primordial germ cells (PGC) and isolated PGC from murine and bovine embryos were examined for in vivo growth and differentiation after transplantation under the kidney capsule of athymic mice. Genital ridges were collected from day 11.5 and 12.5 murine and day 34 and 37 bovine embryos. Murine genital ridges and isolated PGC collected at 11.5 days post-coitus (dpc) and isolated murine PGC collected at 12.5 dpc developed into tumors. Day 34 and 37 bovine genital ridges, but not isolated PGC developed into tumors. The bovine origin of the tumors was confirmed by an analysis of the bovine DNA sequences. Tumors from both species consisted primarily of mesoderm-derived cell types, including connective tissue, cartilage, smooth muscle, fibroblasts, osteoblasts and bone matrix. No detectable ectodermal derivatives were observed in tumors of either species. Undifferentiated stem cells were not detected in the tumors, suggesting that the tumors were benign teratomas. Results of this study demonstrate the pluripotency of bovine PGC by experimental induction of teratomas after xenotransplantation under the kidney capsule of athymic mice. Stimulation of PGC survival and proliferation in an ectopic graft could be useful toward the isolation of pluripotent embryo-derived stem cells.