Prostate specific antigen(PSA) is widely used as a marker for screening and monitoring prostate cancer. However, there are several assay methods, which often give different PSA values for the same patient sample. A possible cause of these discrepancies might be the varied immunoreactivity with free-PSA and complex-PSA among methods. This study revealed that pre-treatment of samples with anti-free-PSA monoclonal antibodies improved polyclonal antibodies-based non equimolar PSA assay to equimolar assay. This might be achieved by that the monoclonal antibodies bind to free-PSA and then change its reactivities with polyclonal antibodies to those similar to complex-PSA.