SUMMARYHybrid offspring from C57BL/6 (B6) females mated to males of the subspeciesMus musculus castaneusreceived B6 skin grafts. No strong Y chromosome-linked histocompatibility genes were detected, although occasional rejection of parental grafts by both male and female hybrids was observed after long periods. Rejection was attributed to interaction of B6 andCastaneus-derived genes in the hybrids.