Eighteen-day-old embryonic femora of Strong A mice were transplanted subcutaneously to adult male mice of Strong A strain as isografts and the CBA strain as homografts. Transplants were observed grossly in vivo for three weeks. Host mice were sacrificed at intervals from five to twenty days after transplantation. Vascular, roentgenographic, and histological studies were carried out. A total of 186 specimens was examined of which sixty were isografts, seventy-one homografts, and fifty-five ungrafted controls.

Both isografts and homografts became vascularized between the fourth and fifth day after transplantation. Subsequently, the vascular supply increased in the isografts but decreased in the homografts. The host bed around the isografts remained clear throughout and became hyperemic then returned to normal, whereas the host bed around the homografts was clear, then cloudy, then hyperemic thereafter.

Histologically, endochondral and intramembranous ossification appeared normal in the isografts (although lesser in degree than the controls). The secondary center of ossification for the femoral condyles appeared at the normal time, but later endochondral ossification decreased until it became inactive. In the homografts the marrow was replaced by loose undifferentiated cells, the epiphyseal disc became disorganized, endochondral ossification was disrupted with cartilaginous rests persisting in the diaphysis, and vascularization of the secondary center of ossification of the condyles was delayed. Infiltration with small round cells was noticeable both inside the grafts, as well as in the host beds surrounding them.

From these results it is concluded that embryonic mouse femur from one inbred strain possesses transplantation antigens that can invoke a tissue immune response in adult mice of a different inbred strain.