It has been reported that lethally irradiated adult B6 mice restored with (A X B6) hemopoietic cells subsequently reject adult A-strain skin grafts because they are not tolerant of A-strain skin-specific (Sk) antigens. This thesis has been confirmed by a series of experiments conducted on neonatally treated animals. Thus sublethally irradiated neonatal B6 mice, inoculated with (A X B6) lymphoid cells, permanently accept these cells while subsequently rejecting adult strain A skin grafts. Further evidence that the destruction of these grafts results from the reaction of the host to Sk antigens, is provided by the fact that similarly treated recipients often permanently accept neonatal A-strain skin. Such grafts usually induce tolerance of adult A-strain skin grafts.