The mucin-like carcinoma-associated antigen (MCA) enzyme immunoassay was tested in 962 healthy controls. MCA levels were compared with CA 125 in 70 patients with benign and 76 with malignant ovarian tumors. In addition MCA was compared with CA 15.3 in 58 patients with breast cancer and with CEA in 50 patients with colon carcinoma. In healthy controls the 95th percentile cutoff of 19.2 U/ml appeared to be higher than generally used. With the common cutoff value of 14 U/ml, a 38% sensitivity and 100% specificity was reached in malignant versus benign ovarian tumors. In colorectal cancer only 4% of patients had elevated MCA serum levels (CEA: 50%). In breast cancer patients the MCA assay performed better than CA 15.3 although only 17.2% showed elevated levels (CA 15.3:7.4%). Thus MCA seems to be of limited value in the diagnosis and follow-up of adenocarcinomas of breast, ovary or colon.