Survival of 436 ABO-compatible skin grafts exchanged in 97 Caucasian families was prolonged if donor and recipient were genotypically, as compared with phenotypically, HLA identical. Among skin grafts between haploidentical family members, a mismatch at the A locus was equivalent to a mismatch at the B locus. Skin grafted from child to mother survived longer than did skin grafted between other family members, other variables being equivalent. A highly significant positive correlation was found between the age of recipient and skin graft survival. In addition, a significant interaction was found between the relationship of donor and recipient and degree of antigen match.