Because Forssman antigen, one of the most well-known heteroantigens, has been noted in certain cancerous tissues, it would seem that the serum levels of the Forssman antibody of patients with these cancers would be low, owing to the absorption of the naturally occurring antibody by the Forssman antigen-containing cancerous tissues. This hypothesis was tested by researchers assessing the serum hemolysin titers of 174 patients with cancer (gastric cancer, 100; colonic cancer, 40; and other cancers, 45) and of 856 age-matched, sex-matched, and blood type-matched healthy individuals against the serum levels of sheep red blood cells. Serum levels of the hemolysin of patients with gastric cancer tend to be lower than those of patients with other types of cancer and also lower than those of age-matched and sex-matched controls. The decrease was especially prominent in patients with moderately differentiated adenocarcinoma of the stomach. Preoperative and postoperative serum samples of 40 patients with gastric cancer were analyzed therefore to determine the effect of surgically removing the cancer on the serum level of the hemolysin. The results showed that the serum levels of the hemolysin antibody increased in all 40 patients after the successful surgical removal of the cancer. However, in patients with recurrence of the cancer, the serum levels of hemolysin decreased again in 11 of 11 patients. These results indicate that the serum levels of the Forssman-like hemolysin could be used as an index of recurrence in patients with gastric cancer after surgical removal of the cancer.