A mucin-like carcinoma associated antigen (MCA), which is recognized by the monoclonal antibody b-12, was found to be elevated in sera of breast cancer patients. Since an immunohistochemical reaction of the monoclonal antibody b-12 was found in epithelial tumors of the ovary we investigated MCA serum levels in 50 patients with ovarian cancer (mean age 59 years, range 31-81 years). In addition, CA 125, CA 19-9 and CEA were determined to compare sensitivity, specificity and the predictive value of the positive test of each parameter used in this study. Blood samples were obtained in 20 patients with progressive disease and in 30 patients during disease free intervals. The MCA serum levels of patients with progressive ovarian cancer (mean +/- SD: 14.7 +/- 14.6 U/ml) did not differ significantly from those of patients in remission (mean +/- SD: 8.2 +/- 5.3 U/ml) or from values of a healthy control group (mean +/- SD: 7.7 +/- 3.8 U/ml, n = 70). Women with progressive disease displayed significantly higher CA 125 (p less than 0.0001) and CEA (p less than 0.0063) serum levels than patients in remission. No significant difference was found for CA 19-9 in patients with ovarian cancer, irrespective of the clinical status. Considering marker surge and tumor progression, the highest sensitivity was found for CA 125 (75%). Sensitivities of the other markers were significantly lower and reached only 25-35%. The predictive value of elevated marker levels as well as specificity of the marker substances were similar. Sensitivity could be extended to 90% if elevation of CA 125, CA 19-9, CEA and MCA were taken into consideration, however specificity was lowered by using this marker combination.