et al. Prostate-specific antigen in serum occurs predominantly in complex alpha 1-antichymotrypsin. Clin Chem

Prostate-specific antigen (PSA),3 a kallikrein-like serine protease, is one of the most abundant proteins secreted by the prostate into seminal fluid (1). In the early 1990s, when the study reported in the paper pre-sented here was initiated, serum testing for PSA was coming into widespread use as a means of testing for prostate cancer. Nevertheless, several aspects of PSA in serum remained puzzling; notably, different assays yielded up to 4-fold differences in measured serum PSA concentrations, despite standardization against PSA from seminal fluid. Before our investigation, PSA in seminal fluid was known to occur as activemonomeric peptidase (1), but the nature of PSA in blood remained unclear.We spec-ulated that release of catalytic PSA into an abundanc