Myeloperoxidase-catalyzed oxidation of chloride (Cl-) to hypochlorous acid (HOCl) resulted in formation of mono- and dichloramine derivatives (RNHCl and RNCl2) of primary amines. The RNCl2 derivatives could undergo a reaction that resulted in incorporation of the R moiety into proteins. The probable mechanism was attack of RNCl2 or an intermediate formed in the decomposition of RNCl2 on histidine, tyrosine, and cystine residues and on lysine residues at high pH. Incorporation of radioactivity from labeled amines into stable, high molecular weight derivatives of proteins was measured by acid or acetone precipitation and by gel chromatography and electrophoresis. Whereas formation of RNCl2 was favored at low pH, the subsequent incorporation reaction was favored at high pH. Up to several hours were required for the maximum amount of incorporation, which was less than 10% of the label in RNCl2. For the amines tested, incorporation was in the order histamine greater than 1,2-diaminoethane greater than putrescine greater than taurine greater than lysine greater than glucosamine greater than leucine greater than methylamine. Initiation of the reaction required HOCl, and oxidized forms of bromide, iodide, or thiocyanate did not substitute. Inhibitors of incorporation fell into three classes. First, ammonia or amines competed with the labeled amine for reaction with HOCl, so that larger amounts of HOCl were required. Second, readily oxidized substances such as sulfhydryl or diketo compounds or thioethers (methionine) reduced RNCl2. Third, certain compounds competed with protein as the acceptor for the incorporation reaction. The amount required to block incorporation into protein depended on protein concentration. Among these inhibitors were imidazole compounds (histidine), phenols (tyrosine), and disulfides (glutathione disulfide, GSSG). Low yields of derivatives of histidine, tyrosine, and GSSG were detected by thin-layer chromatography. Acid-precipitable derivatives were obtained by reacting RNCl2 with polyhistidine or polytyrosine, and to a lesser extent with polylysine at high pH, but not with other poly(amino acids). Precipitable derivatives were also obtained by incubating MPO-containing extracts from leukocyte granules with hydrogen peroxide, Cl-, and labeled amines. The extracts were found to have a high content of substances with primary amino groups, which competed for incorporation. The results account for oxidative incorporation of amines into proteins in leukocytes and provide evidence that HOCl and nitrogen-chlorine (N-Cl) derivatives are formed in these cells. The characteristics of the incorporation reaction suggest that it would not contribute significantly to the antimicrobial activity of myeloperoxidase (MPO). Nevertheless, the reaction may provide a sensitive method for studying MPO action in vivo.