Abstract: A method for determining the complexed forms of immunologically determinable prostate specific antigen (cPSA) in a blood sample, e.g., by two-site immunometric assays, in which the blood sample is treated to render free PSA (fPSA) immunologically nondetectable. A particularly preferred immunometric assay method employs three anti-PSA antibodies: an antibody that binds to both cPSA and fPSA (anti-tPSA), a second anti-tPSA antibody which is characterized by the unique property that binding to fPSA is blocked by binding of fPSA-specific antibodies, and a third antibody which is a fPSA-specific antibody. Thus, binding of the fPSA-specific antibody to PSA in the sample allows only cPSA to be measured in the immunometric assay. Measurement of cPSA blood levels has been found to provide a method for aiding in the diagnosis and monitoring of prostate cancer that is highly sensitive and specific, and eliminates the need for a significant number of patients to undergo unnecessary prostate biopsy.