We have incorporated commercially available CEA standard and antiserum into the triple isotope double antibody radioimmunoassay and we have evaluated this assay for the routine determination of CEA. The competitive protein binding (CPB) assay for CEA can be performed directly on serum or plasma without perchloric acid extraction. The assay sensitivity was 0.98 ng/ml, and the day-to-day precision as defined by the coefficient of variation was 12.5% and 13.3% for mean values of 7.6 and 23.9 ng CEA/ml, respectively. The normal range (X +/- 2 S.D.) for CEA determined with the direct CPB method was 3.2--6.2 ng CEA/ml for non-smokers. The upper limit of normal for smokers was 10.0 ng/ml. A method comparison study (Roche perchloric acid extraction vs. direct CPB) showed excellent agreement between the methods for plasma samples containing less than 20.0 ng CEA/ml. The least square analysis parameters were: N = 116, slope = 1.01, y-intercept = 3.5 ng/ml, Sy/x -2.05 ng/ml, and the correlation coefficient was 0.79. Recovery and dilution studies showed no demonstrable non-specific interference due to serum proteins in the direct CPB assay. The clinical significance of the direct CPB assay for CEA was assessed by correlating serial CEA values with the clinical status of patients with breast and colorectal cancer. Increasing CEA values correlated with progressive or recurrent neoplastic disease, and decreasing CEA values correlated with response of the patient to therapy. No false positive direct CPB values for CEA were observed in the clinical study or in the method comparison study. Our laboratory and clinical evaluation demonstrate that the direct CPB method is an accurate and reliable method for the quantitation of CEA. In addition, the method permits high volume analysis and eliminates the hazards to safety that are associated with perchloric acid.