Immunological studies were performed to determine the cancer-specific fractions in the sera taken from the gastric cancer patients. The antigen, which used in this studies, was the pooled sera from the advanced incurable gastric cancer patients and the antisera were from the rabbits sensitized with the intramuscular injection by Freund's complete adjuvant. 1. With immunoelectrophoretic examination, the increased density of the precipitate lines was demonstrated in α-and β-globulin portions in the majority of the patients. however, it had no definite proportional relation to the development of cancer. 2. Density of precipitate lines maintaned its level up to the period of 4 to 5 weeks after extended radical operation for gastric cancer. It showed remarkable increase immediately after the operation, unexpectedly. 3. There was little difference in the pattern of the cancer serum and the non-cancer by means of the plate method of Ouchterlony.