SUMMARYThe production of tissue-typing reagents for dog leucocytes by allografting procedures is described. Skin and small bowel grafts were efficient in producing informative sera. A negative correlation was found between the duration of graft survival and the subsequent development of cytotoxic antibodies. It was shown that these reagents were useful in the selection of littermate donor-recipient pairs for transplantation experiments. Evidence presented from family testing and tissue transplantation supports the existence of a major (DL-A) histocompatibility locus in dogs