Skin grafts were made on inbred rabbits varying in number of generations of inbreeding. For an average of 6, 12, and 16 generations of sib-mating, the percentages of graft acceptances were 12.5, 21.6, and 43.7 respectively. The number of loci with allelic differences was estimated for the parents at zero generation, for the different inbred lines. No females permanently accepted grafts from their male sibs, but the average survival of 106 days was much longer than that for grafts between inbred lines. Natural selection in favor of heterozygosis was discussed. © 1968 by The Williams and Wilkins co.