SUMMARYParabiosis of pregnant Wistar strain female rats with Long-Evans male rats produced a state of immunological tolerance to Long-Evans homografts in the offspring. Animals grafted with skin between 3 and 8 weeks after birth retained their grafts for more than 2 months (mean survival, 97 days), while control animals rejected theirs within the 2nd week. Large scale transplacental transfer of donor strain cells can be ruled out as an explanation of the phenomenon because there was no sign of runting in the tolerant animals. It seems possible that humoral factors played a part.