A colony of dogs was expanded by selective breeding to study the immunogenetic determinants coded for by the major histocompatibility complex (DLA). Polymorphic determinants were identified by alloantisera specific for DLA-A and B loci antigens and by the mixed lymphocyte culture (MLC) which defined alleles at the D locus. Thirteen families totaling 58 offspring were produced and typed for allelic determinants coded for by each of the three gene loci. Allelic segregation in a codominant manner occurred as expected and a recombinant between the A and B loci was detected. A number of animals were homozygous at one or more loci, thus providing genetically standardized animals as a source of typing cells, antigens, and sera to further study the immunogenetic details of DLA and for in vivo studies in transplantation biology.