The objective of the present study was to provide a morphological characterization of spermatogenesis following germ cell transplantation into the seminiferous tubular lumen of another mouse. The recipient mice (W-locus) were sterile because of a defect in spermatogenesis resulting from the failure of virtually all germ cell precursors to migrate to the genital ridge during embryonic development. Recipient mice containing intratubular injections of testis cell suspensions from C57 mice were allowed to develop for over 1 year, whereupon animals were sacrificed and testis tissue examined by light and electron microscopy. Donor mouse cells formed normal cell associations (stages) as viewed in cross-sectioned tubules. Spermatogonia were found exclusively in the basal compartment, indicating that they were translocated from the tubule lumen through the Sertoli cell junctions, eventually to reside on the basal lamina. Some tubules looked entirely normal from both a quantitative and qualitative standpoint. Others showed qualitative and quantitative impairment. In some tubules a generation of cells was missing from a cell association. A variety of degenerating cells and structural abnormalities were responsible for this impairment, however, the most common abnormalities were seen during the elongation phase of spermatogenesis. Elongation abnormalities and the subsequent degeneration of these cells led to the presence of fewer-than-expected elongate spermatids. There were regions of the testis where no spermatogenesis was noted and only Sertoli cells were present. These regions were generally typical of the testis histology seen in animals not exposed to injected germ cells. However, Sertoli cells in these regions phagocytosed sperm produced in spermatogenically active regions of the tubules. Because transplantation of germ cells, either from fresh or from frozen cells, had wide-ranging implications in biology and medicine, characterization of spermatogonial transplants is an important step in improving this procedure.