Rejection of skin homografts in larvae of Rana pipiens

Larvae of the leopard frog, Rana pipiens , regularly reject skin homo‐grafts. Grafts from sibling larvae show initial signs of necrosis later than grafts from non‐siblings; grafts between very young feeding larvae also show initial necrosis later than those between older larvae. Homograft rejection pattern is essentially the same in larvae of stage IV and older, in hypophysectomized larvae, and in larvae immersed in thyroxine for two weeks after grafting. Second grafts from the same donor are necrotic 4 to 5 days before first grafts are. The conclusion is drawn that destruction of skin homografts in frog larvae is independent of metamorphic phenomena, and is an immunologic reaction, possibly involving the thymus gland. The weakened response of very young larvae may reflect an immunologic mechanism that is not yet mature.