Objective - the study of hypochlorous acid (HOCl) and its derivatives production, which catalyzed by human neutrophil myeloperoxidase, using “turn-on” fluorescent sensor - celestine blue B. Materials and methods. Neutrophils were isolated from the venous blood of healthy donors. Phorbol 12-myristate 13-acetate, N-formyl-methionyl-leucyl-phenylalanine, plant lectins, HOCl-modified proteins were used as agonists. N-acetylcysteine, 4-aminobenzoic acid hydrazide, isoniazid and ceruloplasmin were used as regulators of neutrophil myeloperoxidase activity and/or HOCl scavengers. Results. Using a wide range of agonists and inhibitors, it has been shown that celestine blue B is oxidized in vitro by HOCl and its derivatives as a result of neutrophil myeloperoxidase activity. The oxidation of celestine blue B by HOCl-modified human serum albumin (HSA-Cl) and inhibition of this process by monoclonal antibody against HSA-Cl (IgM class) was also found. Conclusion. Based on the developed method using celestine blue B, it is