1. The plasma of guinea pigs treated by injections of rat sarcoma exhibits a toxic action in tissue culture preparations on the cells of both rat sarcoma and rat embryo skin. Similarly, the plasma of guinea pigs immunized by injections of rat embryo skin is toxic for cells of both types. 2. Injection of rat blood immunizes against both sarcoma and embryo skin, although not so strongly as injections of the two tissues. 3. Guinea pigs receiving injections of either chick embryo heart or intestine develop cytotoxic substances for both of these tissues. 4. The preceding findings tend to show that cytotoxins formed after the injection of different body tissues into a foreign species are to no extent specific for the tissues injected.