SummaryA study was made of the progeny of a pair of dizygotic twin bulls whose blood group phenotypes were identical but were genotypically different, indicating a complete replacement of one of the bulls haematopoietic tissus by that of its cotwin. The study was made in an attempt to determine whether the genes of one sibling were transmitted by the other. If it were so it would follow that the germ cells of the testes were chimaeric. Blood typing of 97 progeny provided no evidence to indicate the presence of such a germ cell chimaerism.