The kinetics and mechanism of the photoinduced electron-transfer reaction between cytochrome c and zinc-substituted myoglobin, the latter of which lysine residue was modified with diethylenetriaminepentaacetic acid, were studied in an aqueous solution at 25 degrees C, pH = 7.0 (a 10 mM phosphate buffer) and ionic strengths from 0.02 to 0.32 M (NaCl). The excited triplet-state of the modified zinc myoglobin was efficiently quenched by the oxidized cytochrome c within a diprotein complex. Both the intracomplex electron-transfer quenching rate constant and the binding constant of the diprotein complex decreased with increasing ionic strengths.