The CA 15-3 RIA kit (Centocor Co.) based on a solid-phase radioimmunoassay was evaluated both experimentally and clinically. The results of a basic study of the reproducibility, recovery and dilution were very satisfactory. Serum levels of CA 15-3 were determined in 211 patients with malignancies including 13 breast cancers, 81 patients with benign diseases and 13 controls. Elevated levels of this antigen (greater than 25 U/ml) were observed in 16.7% of esophageal cancers, 9.7% of gastric cancers, 7.9% of colon cancers, 10% of hepatocellular cancers, 10% of biliary tract cancers, 10% of pancreatic cancers, 33% of lung cancers, and 75% of recurrent breast cancers. Thus, in malignant diseases the positivity was low except for recurrent breast cancer. On the other hand, the false-positive rate in benign diseases was only 3.7% and the false-positive cases had a level of 40 U/ml or less. These results indicate that the serum levels of CA 15-3 may have diagnostic value in patients with recurrent breast cancer.