The milk factor of mammary mouse carcinoma is highly antigenic and stimulates the formation of antibodies in both rabbits and rats. The mouse cancer antiserum neutralizes and renders inactive mouse cancer centrifugates. Equally effective anti-serums are prepared from spontaneous and from transplant tumors. Normal rabbit and rat serums have a slight effect in inactivating the mouse cancer virus. The finding that anti-serum against normal mouse tissue does not have any neutralizing or adverse effect on the milk agent indicates that the agent is a virus of exogenous origin.