Tooth germs (first molars) of 4–5-day-old mice were transplanted to the s.c. connective tissue of adult recipients in the following strain combinations: B10 B10. BY (H-1odifference), B10.BY B10 (H-1ddifference), B10 B10.LPa (H-3adifference), B10.LPa B10 (H-3bdifference), B10 B10.129(21M) (H-4adifference), B10.129 (21M) B10 (H-4bdifference), and A.BY B10 (multiple non-H-2difference). The recipients were killed at different intervals from 5 to 280 days after transplantation and the grafts were examined histologically after standard hematoxylin and eosin staining and marking with Procion dyes. In all the single non-H-2differences, the allografts behaved like syngeneic grafts and there was no evidence of immune allograft reaction. After a transient period of adjustment, during which a nonspecific inflammatory response took place, the tooth germs continued to develop and mature. The deviations from normal development could be attributed to the trauma caused by the surgical procedure and to the heterotopic environment of the grafts. Tooth germs transplanted across the multiple non-H-2barrier (A. BY B10), on the other hand, showed evidence of relatively strong immune reaction. They were heavily infiltrated with mononuclear cells and their further development was either completely arrested or seriously distorted.