Prostate specific antigen (PSA) has recently been proposed, with the advent of inoreasingly sensitive assays, as a suitable marker to screen populations for occult prostate cancer. Objective To simplify prostate cancer screening by validating the mensum of PSA from blood dried on filter paper and subsequently sent through the mail. Methods Serum obtained by venepuncture from individuals requesting PSA testing and paired dried blood spots sent separately through the mail were assayed for PSA using either a commercially available, highly sensitive automated assay (Immulhe, third generation PSA) or a ~ssamh assay based on time resolved fluorescence. Results PSA purified as the free 30 kD Ixotein (from seminal plasma) or as the 100,000 kD complex bound to and, ct t-antic.hymottypsin was added to female whole blood to determine its distribution betwcen intracelluisr and extracellular comparm~ents. Concentrations in whole blood were the same as those found in the plasma corrected for hematocrit indicating that PSA remained outside the RBC in the extracellular compartment. We spotted spiked blood onto Schleicher and Schuell #903 filter paper, extracted the dried blood from five 3 MM2 punched-out disks into 6% BSA containing 0.05% Twcen-20 and constructed calibration curves that showed a working assay range between 0.4 and 50 pg/L. Precision experiments conducted on spotted paper gave CV's <10%. Comparison of PSA levels determined between paired serum and mailed dried blood spots showed a consistent relationship. Conclusions This study confirms that sensitive, precise and reliable PSA assays can be performed on dried blood spots on filter paper and may have value in screening programs where patient specimens are sent to the laboratory through the mail.