Lymphocytotoxic isoantibodies were produced in littermate dogs following allogeneic skin grafting. Nonspecific positive reactions in the cytotoxicity test were eliminated if the rabbit serum used as a source of complement was absorbed with dog spleen. The alloantisera showed a pattern of reactions that may be used for typing of individual dogs for degrees of tissue compatibility. A correlation was observed between the graft rejection times and titers of antibodies.