On the basis of optical difference spectra, lactoperoxidase (LPO) was shown to form a 1:1 complex with aromatic donor molecules: resorcinol, hydroquinone, phenol, p-cresol, guaiacol, aniline, and benzohydroxamic acid. As compared with horseradish peroxidase (HRP), the values of the dissociation constant, Kd, of LPO-donor complexes were found to be 4-720-fold larger and were not greatly changed in the presence of KCN and by changes in pH in the range 4-9. The apparent enthalpy and entropy of the binding reactions were found to be -13 kJ mol-1 and -29 J mol-1 K-1, respectively, somewhat smaller (in absolute value) than the corresponding values of HRP. The difference spectra of LPO-donor complexes resembled each other, in contrast to the case of HRP, and the bindings of the donors to LPO occurred in a competitive fashion between the donors. Incubation of LPO with phenylhydrazine and hydrogen peroxide markedly depressed donor binding, the intensity of the Soret band, and the catalytic activity, probably as the result of formation of meso-phenyl derivatives of the heme. These findings suggest that the binding of aromatic donors to LPO occurs at a specific site, probably near the heme edge, where the electron transfer in the peroxidase reaction may take place.