This study investigated the effect of increased phylogenetic distance on the outcome of spermatogonial transplantation, with porcine donors and mice recipients. It was designed to develop a technique for detecting foreign donor cells in recipient animals. Porcine male germ cells were harvested from postnatal male testes and incubated with the lipophilic membrane dye PKH-26. For transplantation, approximately 10(6) PKH-26-labelled porcine male germ cells were injected into the efferent ducts of mouse testes. Animals were sacrificed at post-graft days 1, 10, 30, 45, 60 and 150 (n = 5 each). Serial frozen sections of explanted testes were prepared for detecting labelled cells. Transplanted porcine donor cells were easily detected in the recipient tubules for 8 weeks. After transplantation, we could detect both incorporation into the basement membrane and differentiation of grafted porcine donor cells by our double detection system, using PKH staining and slide PCR. However, our RT-PCR and apoptosis results revealed that most of the grafted porcine male donor cells could not differentiate past early-meiotic spermatocytes. We could induce partial differentiation of xenografted porcine donor cells in mouse testes, but not full induction of spermatogenesis. We have developed a very reliable technique for detecting foreign donor cells in recipient animals using a combination of PKH staining and slide PCR methods. Our results provide a valuable experimental model for applying and evaluating this technology in other species.