Mouse-to-mouse transplants were studied at 10 min, 9 h, 24 h, 1 week, 1 month, 2 months, and 3 months post-transplantation. Data from a previous light microscope study were confirmed and extended using morphometric and ultrastructural techniques. As soon as 10 min after introduction of the germ cells from one mouse into the tubule lumen of a recipient mouse they developed relationships with small Sertoli cell processes. The extent of this surface-to-surface relationship increased in animals sacrificed up to 1 week post-transplantation. Most transplanted germ cells retained the characteristics of the donor germ cells after they had been isolated and pelleted. Nearly all transplanted cells eventually underwent phagocytosis by the recipient Sertoli cells. The presence of small apparent clones of germ cells after 1 week of transplantation indicated that some germ cells may divide and survive for short periods within the epithelium. No discernible qualitative subcellular changes in the host Sertoli cell accompanying the development of transplant spermatogenesis were noted. Macrophages were present in the region of the boundary tissue between myoid cells and appeared to increase in number in the peritubular tissue of transplanted testes. Images suggest that they migrated into the tubule to gain entrance to the lumen and there take on the form of activated macrophages. Some macrophages phagocytose sperm at 2 months and 3 months post-transplantation. A testis weight increase previously demonstrate to occur at 24 h post-introduction of germ cells was found to be due to an increase in the volume of the tubular lumen. The increase of lumen size at 24 h was not related to the volume of the injected material. It is suggested that the presence of injected cells, likely germ cells, in the tubule lumen stimulated increased secretion by the Sertoli cell.