Cellular cytotoxicity generated in vitro has been studied with canine T and B lymphoblasts as target cells. B lymphocytes were isolated by rosetting with protein A labeled sheep red blood cells and stimulated with sepharose bound protein A. Such highly purified stimulated B lymphoblasts appeared to be useful target cells for cellular cytotoxicity. Cold target inhibition studies and limiting dilution analysis revealed that B lymphocytes did not carry targets for cytotoxic T lymphocytes (CTL) which were not present on PHA stimulated T lymphoblasts.