A sensitive sandwich-type enzyme immunoassay has been developed for quantitation of a human prostate-specific antigen (PA). With this method, PA at a concentration as low as 0.10 ng/ml can be detected. The assay was reproducible as within and between assays yielded a coefficient of variation of 5.7% and 4.6%, respectively. Only human prostate tissues (n = 31) were shown to contain PA. No PA was detected in other human normal or tumor tissues (n = 13). PA was not detectable in sera from normal females (n = 17) or female cancer patients (n = 25). A mean +/- S.D. of 0.47 +/- 0.661 ng/ml (ranging from less than 0.10 to 2.6) ws obtained from a group of 51 normal males. Sera from male patients with nonprostatic cancer contained a similar range of PA as that of normal males. Patients with prostate cancer (371 of 442) and benign prostatic hypertrophy (13 of 19) were shown to have elevated levels of circulating PA. Although no quantitative difference in PA levels was found between the benign prostatic hypertrophy group and Stage A of prostatic cancer, patients with Stages C and D prostatic cancer exhibited significantly elevated levels of PA qualitatively and quantitatively. These results therefore indicate that PA is a histiotypic product of the prostate and may be of use as an adjunctive tool in diagnostic procedures of prostate cancer.