A simple method for finding tunneling orbitals of a long-distance electron transfer system is described. The procedure is an approximate biorthogonalization of many-electron donor and acceptor states in which the corresponding orbitals of the system are obtained; one pair of such corresponding orbitals describes the tunneling electron. Usually, the biorthogonalization is performed on the states that describe simultaneously both donor and acceptor complexes and the bridge connecting them, which make the procedure computationally demanding. In this paper a significant simplification of the procedure is proposed. In the procedure, to identify the tunneling orbitals roughly one half of the system is needed-only donor complex, or acceptor complex, plus part of the bridge. The procedure is practically equivalent to biorthogonalization, but computationally much simpler. Applications are illustrated with the results for a model of biological long-distance electron transfer in Ru-modified azurin, and electron transfer involving heme a of cytochrome c oxidase. (C) 2004 American Institute of Physics.