Monoclonal antibody S202 reactive with scirrhous gastric carcinoma cell line MK-01 was used in an immunoperoxidase assay to characterize the antigenic profile of a variety of fixed samples. Representative sections of malignant tissue from the stomach, colon, esophagus, pancreas, bile duct, gallbladder and breast were examined by the avidin-biotin complex immunoperoxidase technique and compared. The antibody reacted with 90 out of 98 stomach adenocarcinomas, and with 30 out of 32 colon adenocarcinomas with diffuse cytoplasmic staining, whereas it reacted with only 19 out of 37 breast carcinomas, and 5 out of 10 esophageal squamous cell carcinomas. The antigenic determinants appeared to be a carbohydrate, since it was highly sensitive to sodium periodate.