The non-heme oxygen carrier hemerythrin (Hr) undergoes autooxidation to give metHr, Fe(III),Fe(III) , a form that will not function in oxygen storage or transport. A methemerythrin reduction system has been isolated from the erythrocytes of the sipunculid Phascolopsis gouldii that reduces metHr to deoxyHr, Fe(II),Fe(II) , the active form in oxygen transport and storage. This methemerythrin reduction system is composed of cytochrome b(,5) and cytochrome b(,5) reductase with NADH acting as the source of electrons;EPR has shown the involvement of a one-electron reduction pathway for metHr in intact erythrocytes. Erythrocytes that have been subjected to stress from lack of oxygen show a (semi-met)(,R)Hr-type signal that progressively grows with time, presumably generated from reduction of metHr;A soluble cytochrome b(,5) was isolated from the erythrocytes of P. gouldii and characterized. The P. gouldii cytochrome b(,5) is very similar to cytochrome b(,5) found in the hemoglobin-containing erythrocytes;A cytochrome b(,5) reductase was also isolated from the erythrocytes of P. gouldii. This enzyme is membrane-bound and was solubilized with 2% Triton X-100, a non-ionic detergent. This enzyme catalyzes the reduction of cytochrome b(,5) by NADH. Use of this enzyme to keep the cytochrome b(,5) reduced completes a metHr reduction system with NADH as the electron donor. The metHr is first reduced to (semi-met)(,R)Hr by the reduced cytochrome b(,5). The (semi-met)(,R)Hr reacts further to give deoxyHr in a manner analogous to the reduction of metHr with inorganic reagents;Cytochrome b(,5) was also isolated from the erythrocytes of Themiste zostericola, showing that cytochrome b(,5) is a common component of hemerythrin-containing erythrocytes;Reduced glutathione has also been detected in P. gouldii erythrocytes. Reduced glutathione is capable of reducing metHr, albeit more slowly than the cytochrome b(,5) metHr reduction system.