The Third International Workshop on Canine Immunogenetics involved 80 potentially DLA-D homozygous typing cells obtained from dogs of various breeds and submitted from five laboratories in Europe and the United States. Mutual reactivity of all cells was studied in mixed leukocyte cultures, and stabilized relative responses were used for analysis. Intralaboratory and interlaboratory comparisons of results suggest that a stabilized relative response of 30% represents an acceptable parameter for "typing responses" indicating phenotypic DLA-D identity of stimulator and responder cells. Using this criterion, 10 clusters of homozygous typing cells were defined and accepted on an international level, and they were assigned the specificities Dw1 to Dw10. At least six additional (provisional) specificities were recognized that were well characterized within individual laboratories but require additional testing before workshop specificities can be assigned. These data show that DLA-D typing is feasible and represents a useful tool in the genetic analysis of the canine major histocompatibility complex. Much work is needed to confirm the present results in family studies, to determine gene frequencies, and to analyze at a molecular level the antigens responsible for mixed leukocyte culture reactivity.