Fifteen dog families were studied in mixed leukocyte culture. In eight families of seven different breeds, one-way nonstimulation in mixed leukocyte culture was observed. This could be explained by lymphocyte-defined (LD) homozygosity in most instances. In one family, an inclusion phenomenon was postulated, and in another, a deletion. LD homozygotes can serve as reference for LD typing and can also help elucidate the relative role of the LD locus for transplantation of organs.