Antigen CA 12 5, associated with ovarian carcinomas, was assayed in the serum by an immunoradiometric method using monoclonal antibody OC 125. In a control group of 67 women in apparent good health, CA 12 5 serum levels in 64 cases (96%) were lower than 35 U/ml, the value arbitrarily considered as the upper normal limit. Using this limit, in benign lesions excluding cirrhosis, 19% (5/27) had raised levels. None of 10 cases with benign ovarian lesions had elevated values. However, in cirrhosis 86% of cases were positive being very high in 43% of cases (greater than 500 U/ml). In 38 cases of ovarian carcinomas, the overall positivity was 92% (35/38); in serous carcinomas it was 96% (26/27). For other non-ovarian cancers, particularly colonic and mammary adenocarcinomas at advanced stages, the positive rate was 43% (19/44). Finally, for 20 patients treated by chemotherapy for ovarian carcinomas and followed up by repeated CA 12 5 assays over a 2 to 8 month period, an increase or decrease in CA 12 5 levels correlated with progression or regression of the disease in 18 cases (90%).