Transplantation of testicular cells, also known as spermatogonial stem cell transplantation, is a relatively new approach in the field of male infertility. We used this technique to determine whether donor-derived sperm production in unrelated porcine recipients is possible following ultrasound-guided transfer of testicular cells. This study was undertaken because we had a strain of Finnish Yorkshire boars with a hereditary recessive gene defect rendering all spermatozoa immotile and anatomically abnormal in homozygous boars. Thus, monitoring of the focal success of colonization of donor spermatogonia with subsequent production of progressively motile spermatozoa was extremely sensitive. Testicular cells from young normal crossbred boars were transplanted into the testes of two boars affected with the immotile short-tail sperm (ISTS) defect. Prior to the transplantations, busulfan was used to suppress recipients' endogenous spermatogenesis. The ejaculates were collected and analysed for the presence of motile spermatozoa. In one of the two recipient boars transplanted with testicular cells from normal donors, motile spermatozoa appeared in the ejaculates 12 weeks after the transplantation. Spermatozoa manually selected under a microscope from a frozen aliquot of ejaculate collected 27 weeks after transplantation were genotyped. In two of the 20 vials the donor-derived genotype was visible. The genotyping results substantiated the success - as indicated by the appearance of motile spermatozoa after the spermatogonial transfer. Thus, donor-derived sperm production in unrelated recipients is possible. In addition, the production after transplantation of progressively motile spermatozoa with normal tail lengths shows that the ISTS defect in Finnish Yorkshire boars apparently results from defective transcription of an essential gene for sperm motility in germline cells. To conclude, the transplantation of donor testicular cells can, at least in boars with the ISTS defect, result in complete focal spermatogenesis.