Based on the observation that adenomatous carcinomas are frequently associated with tumor endocarditis, whereas undifferentiated tumors are not, autopsy material of tumor cases was examined for differences in the immune response with regard to the histological typing of the tumors. In the former group signs of stimulated cellular and humoral immune mechanisms were markedly stronger developed than in the latter group. From these findings it is suggested that tumor endocarditis represents an immune complex disease.