As a novel tumor marker, we employed a quantitative sandwich RIA system utilizing two monoclonal antibodies (115D8, DF3) which react with a circulating antigen expressed by human breast cancer cells. The optimum condition for this assay was found be a 60 minute incubation at 37 degrees C for the first reaction and a 60 minute one at 25 degrees C for the second reaction. Under these optimum conditions, intra-assay variation of control sera was CV 3.6% and inter-assay variation was CV 6.6%. The observed range of CA 15-3 concentration in 75 healthy persons was 7.5 +/- 3.4 units/ml (mean +/- SD) and mean +2 SD was 14.2 U/ml. Less than 15 U/ml was decided as the cut off level. The positive rate in 113 patients with benign diseases was 18%, the serum levels being less than 25 U/ml. Increased CA 15-3 levels in sera of 178 patients were found respectively, in 0%, 41%, 45%, 50% and 75% of stage I, II, III, IV in primary breast cancer and advanced breast cancer. The sera of 10 patients with advanced breast cancer were collected regularly during a 2 to 4 month period. All patients with PD showed increased CA 15-3 levels and four patients in PR showed a clear decrease of serum levels. In 167 other malignancies, increased levels were found in 76%, 63%, 58%, 33%, 32% and 22% of the sera from uterine, pancreatic, ovarian, prostatic, lung and gastric carcinomas. The data revealed that serum CA 15-3 was clinically useful as a tumor marker especially a monitoring marker of advanced breast cancer, but presently the assay is not suitable for the early detection of breast tumors. Also its measurement seemed to be useful in ovarian cancer, uterine cancer, pancreatic cancer and adenocarcinoma of the lung.