PSA-ACT molecular weight is 90 kDa (PSA 28kDa and ACT 60-65kDa). PSA is localised in cytoplasm of prostate acinar cells, ductal epithelium and seminal fluid. PSA exists in 3 major forms; free PSA, PSA-ACT (PSA bound to Alpha-1 Antichymotrypsin) and PSA-A2M (PSA bound to Alpha-2 Macroglobulin). Serum concentration as a free to bound ratio correlates well with tumour mass and clinical stage of prostate cancer. High levels of free PSA can also be seen in serum from patients with benign hypertrophy.