Transport of nuclear proteins into mitochondria is catalyzed by distinct protein translocation machineries in the outer and inner mitochondrial membranes. We have purified the preprotein translocase of the outer membrane of mitochondria (TOM complex) from the fungus Neurospora crassa in amounts that allow detailed characterization (Künkele et al., 1998, Cell 93, 1009-1019). We have determined its subunit stoichiometry, analyzed its function, and examined its architecture using electron microscopy and image analysis techniques. The TOM complex contains a cation-selective high conductance channel. Upon reconstitution into liposomes, it mediates integration of proteins into and across the lipid bilayer. Electron microscopy and image analysis reveal particles with a diameter of about 14 nm. Particles were found with one, two and three pores with a diameter of about 2 nm, which we propose represent protein conducting channels. The pore size fits well with results obtained from planar lipid bilayer sizing experiments using non-electrolyte polymers as molecular probes.