PSA is a chymotrypsin-like serine protease (kallikrein family) produced by the prostate epithelium, and abundant in seminal fluid. PSA can be detected in the sera of patients with prostatic carcinoma. It is predominantly complexed to a liver-derived serine protease inhibitor, alpha-1-antichymotrypsin (ACT). A higher proportion of serum PSA is complexed to ACT in prostate cancer than in benign prostate hyperplasia. Anti-PSA is useful in the positive identification of metastatic deposits of prostatic origin.