The murine monoclonal antibody OC 125 reacts with an antigen (CA 125) common to most nonmucinous epithelial ovarian carcinomas. An assay has been developed to detect CA 125 in serum. By this assay, only 1 per cent of 888 apparently healthy persons and 6 per cent of 143 patients with nonmalignant disease had serum CA 125 levels above 35 U per milliliter. In contrast, 83 of 101 patients (82 per cent) with surgically demonstrated ovarian carcinoma had elevated levels of antigen. In 38 patients with epithelial ovarian carcinoma monitored on 2 to 18 occasions during 2 to 60 months, antigen levels ranged from less than 1 to more than 8000 U per milliliter. Rising or falling levels of CA 125 correlated with progression or regression of disease in 42 of 45 instances (93 per cent). Determination of CA 125 levels may aid in monitoring the response to treatment in patients with epithelial ovarian cancer.