Colicins and phages parasitize outer membrane receptors whose physiological purpose is the transport of metabolites, metals, vitamins, and sugars. From mutagenesis studies, it is known that several colicins require the function of two outer membrane protein (Omp) receptors for cytotoxicity. A formidable list of problems associated with an understanding of a two receptor mechanism for colicin translocation includes the definition of the sites of initial binding and interactions of the colicin with the OM translocator protein, the working lumenal aperture of the translocator, the question of whether the colicin must be unfolded for translocation, the source of energy for unfolding and translocation, the order of colicin translocation, and the sites and mechanism of interaction of the colicins with the Tol-Pal proteins on the periplasmic side of the outer membrane. 3D crystal structures recently obtained of the cobalamin (vitamin B12) receptor (BtuB), and of the complex of BtuB with the 135 residue receptor binding domain (R135) of colicin E3, have provided some new insights on the interactions between two Omp receptors that are necessary for translocation of colicins. Together with spectroscopic data on the R135-BtuB interaction and electrophysiological data on the colicin E3-OmpF interaction, this has led to a proposal for the utilization of two receptors, BtuB-OmpF, in an outer membrane translocon for colicin E3.