OBJECTIVE An assay is described for measuring very low concentrations of prostate-specific antigen (PSA) which could be used to reliably monitor patients with radical prostatectomies potentially to detect early recurrence of prostate cancer. METHODS A combination of immobilized and acridinium ester-labeled monoclonal antibodies was used to develop a two-step, 90-minute chemiluminometric assay. The reference standards for the serum assays were prepared by adding patient sera with a high concentration of PSA to base pools of female sera, which were selected because of low background counts and good recovery of added PSA. The assay was standardized to match the Abbott IMx PSA Assay. RESULTS The serum-based analytic detection limit (calculated as response 2.5 standard deviations [SD] above the zero standard) is 0.004 ng/mL, whereas the "biologic detection limit" (calculated as 2.0 SD above the analytic detection limit) is 0.008 ng/mL. The assay is highly reproducible with interassay coefficients of variation (CV) under 12% down to 0.02 ng/mL, qualifying this as a "second generation" assay (eg, CV < 20% at 0.05 ng/mL). CONCLUSIONS This assay can measure very low concentrations of PSA in plasma and a wide range of PSA concentrations in urine. This assay will provide a valuable analytic tool for the future evaluation of the clinical utility of "ultrasensitive" PSA measurements for the management of prostate cancer.