A new colon cancer antigen is reported. It is designated as COTA, Colon-Ovarian Tumor Antigen, because it is found in mucins produced by both tissues during malignancy. The new antigen was identified by making antibodies against human colon cancer tissue in goats. The antisera were exhaustively absorbed with lyophilized extracts of normal colon, lung, liver, spleen, kidney, plasma, and the well-known colon tumor antigen, carcinoembryonic antigen (CEA). The new antigen was identified by immunodiffusion. Studies of 28 malignant tissue extracts, 10 ovarian adenocarcinoma cyst fluids, 43 normal tissues, and 5 plasma samples revealed that this antigen is found only in colon tumors and mucinous ovarian adenocarcinomas. The antigen was not detected in serous adenocarcinoma of the ovaries, extracts of adenocarcinoma of lung, breast, kidney or stomach nor in the extracts of normal tissues. Other tests show that this antigen is not CEA, Ca 19-9, or CSAp. It is stable to heating at 65 degrees for 5 minutes; it elutes from an ion exchange matrix (DEAE) with 0.3-0.5M NaCl; it migrates to the alpha-2 region on immunoelectrophoresis; and its size, by exclusion chromatography on Sepharose 4B, is 3-15 million daltons. Anti-COTA stains colon cancer tissue sections indicating that COTA is present in goblet-cell mucin.