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The single sulfhydryl group (Cys-50) of the methemerythrin from Phascolosoma gouldii reacts with 5,5'dithiobis(2-nitrobenzoic acid) to form a mixed disulfide. The pulse radiolytically generated formate radical reduces this mixed disulfide to its radical anion. In turn, the disulfide radical anion reduces the protein two-iron center over a nominal distance of 13 A. The rate constant for this intramolecular electron transfer is approximately 15 s-1 at room temperature, pH 7. Between the two redox centers there is an equilibrium driving force of 0.78 V, measured by differential pulsed polarography.