Human prostatic acid phosphatase (PAP) is a non-specific phosphomonoesterase, synthesised and secreted into seminal plasma under androgenic control. Human PAP is a 100 kDa glycoprotein containing two subunits of approximately 50 kDa each. It catalyses the dephosphorylation of organic monophosphate esters, demonstrating optimum activity at an acid pH. Produced by the prostatic epithelium, serum levels of PAP are very low in healthy individuals, but are often elevated in malignant and benign prostatic disease while it has been used as a marker of diagnosis and therapy control of cancer of the prostate gland.