A large magnetic field effect (MFE) up to 13% of the yield of the free radical formation is found in the photoinduced electron-transfer reaction from the tryptophan residue of the hen egg-white lysozyme (HEWL) to the flavin mononucleotide, (FMN). The magnetic field effect is special in the water solution of FMN, and that is reduced to 5-7% by the addition of sodium chloride to the solution and in the system of riboflavin. The MFE up to 13% coincides with the appearance of the fast quenching process of the excited singlet state. The large MFE is attributed to association by Coulomb interaction between the ionic phosphate group of FMN and the cationic surface of the protein molecule.