Modified Boyden's hemagglutination test was performed with the gastric cancer sera and the tannic acid treated human O-type red cells conjugated with the mitochondria fraction from the pooled gastric cancer tissues for the demonstration of cancer-specific humoral antibody. 1. The increase of humoral antibody was recognized in the serum from the gastric cancer patient sensitized with his own cancer tissue. 2. These titers of the humoral antibody were seemed not to be proportional to the development of cancer, the extent of infiltration in stomach serosa and lymphatic metastasis. 3. No significant relatiohship could be noted between these titers of humoral antibody and the immunoelectrophoretic patterns in gastric cancer sera. 4. There was some relationship of inverse proportion between the titers of humoral antibody and the immunoelectrophoretic patterns in the cancer sera before and after extended radical operation for the gastric cancer.