A quantitative immunoassay procedure has been constructed to evaluate levels of ductal carcinoma antigen recognized by murine McAb F36/22. Using this method, 3% of 64 apparently healthy individuals and 13% of 40 patients with benign breast disease expressed serum antigen levels above 70 units/ml. Greater than 50% of 116 patients with clinical evidence of breast cancer demonstrated circulating ductal carcinoma antigen levels above 70 units/ml. Patients with ductal carcinomas of other sites, including prostate and gastrointestinal tumors, also demonstrated elevated levels of antigen (11 and 27%, respectively). The incidence of elevated serum ductal carcinoma antigen levels correlated significantly with the incidence of intratumoral antigen expression. Lectin binding, molecular weight, and density measurements indicated that circulating antigen occurs as a high-molecular-weight glycoprotein with mucin-like characteristics.