BALL-ELSA PSA is a monoclonal radioimmunometric assay kit for detection of serum prostate specific antigen (PSA) developed and generally used in Europe. Basic and clinical study of the kit was performed for evaluation of it's utility in Japanese patients. The sera from 56 patients with benign prostatic hyperplasia (BPH), 35 patients with prostate cancer (PCA) and 18 normal males were examined. Other kits such as EIKEN PSA and EIKEN PAP (prostatic acid phosphatase) were also evaluated in same sera. The results of the range in the measurement, within-assay error, between-assay error, dilution test, recovery test and others were well satisfied. In our clinical study, mean + 2SD of serum PSA values in normal males was 2.57 ng/ml. Serum PSA values determined by the two RIA kits, BALL-ELSA and EIKEN, showed a good correlation (r = 0.9909), but the BALL-ELSA PSA kit yielded values about 2.4 times higher than the EIKEN PSA kit on the same sample. We think that the difference might be largely due to differences in assigned PSA calibrators and diluents between the assay kits. Further investigations and discussions were expected on this point. In our study, the most suitable cut-off level for distinguishing PCA from BPH in BALL-ELSA PSA kit was 10.0 ng/ml.