A selective breeding program based on serological techniques was used to establish three herds of miniature swine, each homozygous for a different allele at the major histocompatibility complex, termed MSLA. By selective immunization of these swine, high-titered antisera detecting the products of individual MSLA alleles were raised and used to confirm the genotype of offspring in these herds. Techniques for performing one-way mixed leukocyte cultures in these pigs were developed and demonstrated the presence of a strong mixed leukocyte culture locus (or loci) closely linked to the serologically defined MSLA locus. Cell-mediated cytotoxicity assays based on 51Cr release were developed to monitor the course of cellular immunity following allotransplantation and exhibited specificity identical to that predicted by the serological typing.