A solid phase radioimmunoassay for human prostatic acid phosphatase has demonstrated substantially greater biochemical sensitivity than a standard enzymatic method for which p-nitrophenylphosphate was used as substrate. Preliminary data indicate that the radioimmunochemical approach can precisely classify 43% stage I-II and 94% stage III-IV prostate cancers. In contrast, the standard enzymatic methods correctly classified only 9% stage I-II and 46% stage III-IV cancers. It is clinically apparent that a radioimmunochemical approach for the measurement of human prostatic phosphatase may have distinct potential in the clinical diagnosis of prostate cancer.