In this study, various analytical aspects of the determination of serum prostate-specific antigen are described as applied to the Abbott IMx PSA and the Hybritech Tandem-E PSA assays. We used mainly specimens from a prostate cancer screening study in progress. A very good comparability between the assays proved to exist in our hands. The long-run variation (16 months) was also rated as acceptable, both for the IMx and the Tandem-E method. The method of choice, Tandem-E, showed good reagent stability over this period. We found, however, a difference in accuracy (Tandem-E +/- 8% higher values) that could not be explained by comparison with Tandem-R.