The fate of split thickness skin allografts between and among major histocompatibility complex (MSLA) homozygous herds of miniature swine has been examined. Among animals matched for all known blood groups, MSLA‐identical skin grafts survived 11.8 ± 0.89 days, while skin grafts from animals differing for one or two haplotypes survived 7.0 ± 0.36 days. Cytotoxic antibodies invariably appeared in the serum of animals rejecting MSLA‐nonidentical grafts, and were never detected among animals rejecting MSLA‐identical grafts.

Among the homozygous DD herd, it was possible to assess the effect on skin allograft survival of a defined difference at the A‐O blood group locus. Skin grafts between DD animals identical at the A‐O blood group locus. Skin grafts between DD animals differing at this locus survived 7.66 ± 0.54 days. Blood group O animals rejecting skin from blood group A animals produced a low liter of antibodies which were cytotoxic for donor lymphocytes. These antibodies were at least partially absorbable by donor red cells. An absorption analysis using other blood group‐defined animals of these herds showed that ability to absorb activity segregated with the A blood group.

These studies confirm the expectation that MSLA is a major histocompatibility complex of miniature swine, having important implications for the outcome of allografts. In addition, they show that the A‐O blood group, or a locus closely linked to it, is also a histocompatibility locus in pigs, and must be respected in allografting studies.