By means of SRBC lysis in agarose gel and Forssman (F) liposome lysis assay, serum levels of F antibodies were determined in various cancer patients. F antibody titers of the patient's sera were shown to be lower than that of normal controls. Following radical surgery, F antibody titers were elevated significantly in most of the patients with advanced gastric, colorectal and lung cancer. Similar tends of anti-A antibody titers were observed in group O and B patients with advanced cancer.