1. The ability of carnosine and carnosine-related compounds (CRCs) to interact with several free oxygen radicals is analyzed. 2. Carnosine, the CRCs (imidazole, histidine, anserine), and ergothioneine were found to be equally efficient in singlet oxygen quenching. During generation of hydroxyl radicals from hydrogen peroxide in the Fenton reaction, carnosine was found to be more effective than the CRCs tested. 3. By measuring the chemiluminescence produced by carnosine and CRCs in rabbit leukocytes in the presence of luminol or lucigenin, we conclude that carnosine and other CRCs play a stimulating role in superoxide oxygen production while suppressing the myeloperoxidase system. 4. ADP-induced aggregation of human platelets is slightly stimulated by carnosine but is inhibited by acetylanserine. 5. The following rank order of efficiency of CRCs was demonstrated while measuring the oxidation of human serum lipoproteins: acetylcarnosine < acetylanserine < homocarnosine = ophidine < carnosine < anserine. 6. The results obtained demonstrate that metabolic transformation of carnosine into CRCs in tissues may play an important role in regulating the native antioxidant status of the organism.