This paper describes formation of complexes of ceruloplasmin (CP) with such proteins of the serprocidin family as azurocidin (CAP37), neutrophilic elastase (NE), cathepsin G (CG), and proteinase 3 (PR3). We present evidence that serprocidins form complexes with CP at a molar ratio 1 : 1. Phenylmethylsulfonyl fluoride, a serine protease inhibitor, did not prevent the interaction of serprocidins with CP in the course of SDS-free disc electrophoresis. CP affected the activities of NE, CG, and PR3 as a competitive inhibitor with K(i) ~ 1 μM. Inhibitory effect of CP depended on ionic strength of the solution and was negligible at NaCl concentrations above 300 mM. In the mode of competitive inhibitors serprocidins suppressed oxidase activity of CP towards p-phenylenediamine. CAP37 displayed the strongest inhibitory effect (K(i) ~ 20 nM). Upon adding various serprocidins to human, rat, rabbit, dolphin, dog, horse, and mouse plasma only CAP37 would form a complex with CP. Synthetic peptide RKARPRQFPRRR (5-13, 61-63 CAP37) displaced CAP37 from its complex with CP. Adding CAP37 to the triple complex formed by CP, lactoferrin, and myeloperoxidase resulted in displacement of the latter from the complex. The dissociation constant of CAP37 with immobilized CP was 13 nM. Therefore, among serprocidins CAP37 can be regarded as the specific partner of CP.