The ability of the erythrocyte membrane to transfer the reducing equivalent from the outer solution into the cells was studied. Erythrocyte hemoglobin transformed into the metstate serves as the electron acceptor. The donors develop during illuminating with visible light the outer solution, containing NADH and eosin. Some precautions were made to inhibit the migration of the donor molecules through the membrane. The photoreduction of hemoglobin in erythrocytes in such conditions can be attributed to the diffusion of some lipophilic electron carriers if they exist in the membrane, or to the ability of the transmembrane proteins to mediate the electron transfer from definite donors to the acceptors.