The peroxyl radical location in horse heart myoglobin (Mb) oxidized with H2O2 was examined by 9-287 GHz electron paramagnetic resonance (EPR) spectroscopy along with the radicals formed upon oxidation of tyrosine (Tyr), tryptophan (Trp), and histidine (His) individual amino acids. X-ray crystallography of the horse heart Mb has revealed that Tyr103, Tyr146, Trp14, and His64 residues are in the vicinity of the Mb heme group and thus could be implicated in Mb oxidation. The 9 GHz axial EPR signal of the Mb peroxyl radical (g(parallel to) = 2.040 +/-0.003 and g(perpendicular to) = 2.001 +/- 0.001) is nearly identical with those of the radicals produced by irradiation of the individual amino acids in the presence of H2O2 (77 K). The 95-287 GHz measurements were sufficient to resolve the individual g tensor components of the Mb, Tyr, Trp, and His peroxyl radicals which cannot be distinguished at 9-35 GHz. The high-field EPR spectra of the Mb peroxyl radical were simulated using a Hamiltonian that describes the exchange and dipolar interaction between an oxoferryl iron and a protein radical. The exchange-coupled oxoferryl-protein radical pair could be best fitted with g values of g(X) = 2.0356, g(Y) = 2.0083, and g(Z) = 2.0024 for the radical, a dipolar coupling corresponding to a distance of 8.3 Angstrom between the radical and the oxoferryl iron, and an exchange coupling (J) of 0.476 GHz. The determined g tensor and the distance related to the Tyr residue at position 146. Tyr146 is assumed to be the most likely candidate for the peroxyl radical location in oxidized Mb.