Schiff bases condensible at pH 7.0-10 after prolonged incubation with neighbouring histidine residues to yield cyclic compounds absorbing at 330 nm are formed by means of four molecules of pyridoxal-5'-phosphate (PLP) interacting with epsilon-NH2 groups of lysine in bovine serum albumin. The condensation is accelerated in the light. The number of PLP molecules interacting with imidazole rings of histidine in protein remains invariable in alkaline medium, in 6-8 M guanidine. At pH 4.0-4.5 (N-F transition) one more lysine residue neighboring to histidine becomes exposed. The splitting of histidine imidazole ring by diethylpyrocarbonate, photoxidation by singlet oxygen stop the formation of the PLP cyclic compounds with protein.