Human prostatic acid phosphatase (ACPP) has been used as a diagnostic marker for prostate cancer. It is synthesized under androgen regulation and secreted by the epithelial cells of the prostate gland. We have confirmed the previous assignment of the ACPP gene to chromosome 3 by probing a panel of 25 human-Chinese hamster somatic cell hybrids, and we have further localized the ACPP gene to chromosome 3q21-q23 by fluorescence in situ hybridization.