Myeloperoxidase (MPO) and eosinophil peroxidase (EPO) are involved in the development of halogenative stress during inflammation. We previously described a complex between MPO and ceruloplasmin (CP). Considering the high structural homology between MPO and EPO, we studied the latter's interaction with CP and checked whether EPO becomes inhibited in a complex with CP. Disc-electrophoresis and gel filtration showed that CP and EPO form a complex with the stoichiometry 1:1. Affinity chromatography of EPO on CP-agarose (150 mM NaCl, 10 mM Na-phosphate buffer, of pH 7.4) resulted in retention of EPO. EPO protects ceruloplasmin from limited proteolysis by plasmin. Only intact CP shifted the Soret band typical of EPO from 413 to 408 nm. The contact with CP likely causes changes in the heme pocket of EPO. Peroxidase activity of EPO with substrates such as guaiacol, orcinol, o-dianisidine, 4-chloro-1-naphtol, 3,3',5,5'-tetramethylbenzidine, and 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonate) is inhibited by CP in a