In the genesis and mutual transformations of reactive forms of oxygen a significant part is played by enzymes. The authors present a list of these enzymes and emphasize their mutual relations. In more detail the following enzymes are described: NADPH-oxidase, myeloperoxidase, nitroxide synthase, xanthine oxidase. These enzymes are discussed according to a uniform pattern which comprises the catalyzed reaction, a description of the molecular structure of the enzyme, its content in different tissues, control of the activity of the enzyme, the importance of the enzyme for the organism, methods of assessment of its activity, clinical sequelae of deficient enzyme synthesis and reasons for an increase or decrease of its activity.

        Key words: free radicals, reactive oxygen species, NADPH-oxidase, myeloperoxidase, nitroxide synthase, xanthine oxidase.