A human monoclonal antibody, YJ-37 (IgM) was generated through the fusion of human B lymphoblastoid cell line HO-323 with the regional lymph node lymphocytes from a colonic cancer patient who was treated with a local immunotherapy. This antibody was purified and conjugated with biotin, after which direct immunohistochemical staining was performed. The results revealed that YJ-37 selectively reacted with colonic cancer (7/19), gastric cancer (3/6), endometrial cancer (1/2) and colonic adenoma (7/13), but not with normal epithelia. Membrane immunofluorescence and FACS analysis also showed that YJ-37 bound to tumor cell surfaces. Furthermore, the chemical structure of the antigen defined by YJ-37 was analyzed by means of thin-layer chromatography immunostaining and ELISA. The results indicated that YJ-37 reacted with sialylated lacto-series carbohydrate chains, which have been reported to accumulate in cancer cells.