From June 1, 1984, to May 31, 1985, 98 cases of epithelial ovarian carcinomas were assessed and followed prospectively using a new murine monoclonal antibody OC 125 which detects the antigen CA 125. Serous tumors comprised 43.7% of cases, mucinous tumors 20.4%, endometrioid tumors 16%, and other epithelial tumors 19.4%. Tumors of low malignant potential and benign epithelial cystadenomas were not included. For this study the upper limit of normal for CA 125 was 20 U/ml. Thirty-six were new cases. In this group the initial CA 125 levels >20 U/ml, >35 U/ml, and >65 U/ml were 97.2, 94.4, and 86.1%, respectively. When mucinous types were excluded the specificity rate did not change significantly. There was no significant difference in initial CA 125 levels between early stages I and II and late stages III and IV. No correlation between tumor bulk and the serum level of antigen was observed. The remaining 62 patients were being followed and in this group 50 were considered to be in remission. Six cases in the remission group had elevated CA 125 levels >20 U/ml and 5 of these developed clinical recurrence. The correlation between the clinical status and concordant fluctuations in the serum levels of CA 125 in all histological types was 87.8 and 93.5% when 10 cases of mucinous tumors were excluded. The contingency coefficient was 0.746. Seven SLOs were performed. All had CA 125 levels <20 U/ml and the mean was 6.9 U/ml. Only 1 case was positive with microscopic disease. In our experience CA 125 was invaluable in the management and follow-up of patients with ovarian carcinoma especially for the early detection of recurrent disease and for the monitoring of patients on therapy.