This invention concerns an antibody which binds with high affinity to human single-chain intact, i.e. not internally cleaved, mature and/or zymogen forms of prostate specific antigen (SCINT PSA). The antibody does not bind to a nicked PSA (PSA-N), wherein said PSA-N has been formed by internal peptide bond cleavage(s) of SCINT PSA resulting in two-chain or multi-chain PSA. This invention further concerns an immunoassay and a method for differentiating patients with cancer of the prostate (PCa) from patients with benign prostatic hyperplasia (BPH) and/or healthy male subjects without PCa, patients with aggressive PCa from patients with indolent PCa and/or patients with clinically localized and/or organ confined PCa from patients with extraprostatic extension of PCa and/or PCa with metastatic spread to lymph nodes or bone marrow using said antibody.