Consistent and rapid bone marrow grafts were produced in lethally irradiated dogs by infusions of marrow cells and peripheral blood leukocytes obtained from single living donors. Canine histocompatibility typing by lymphocyte cytotoxic antisera was evaluated in selecting donor-recipient pairs of unrelated dogs for marrow grafting. Significantly longer survival was observed in recipients that received marrow grafts from matched donors (P> 0.01 < 0.02).