Polystyrene fractions of known narrow molecular weight ranges have been used to characterise methylated Sephadex G-75 and G-100, which have previously been shown to separate the protein components of mitochondrial membranes dissolved in chloroform-methanol. The membrane proteins behave as compact globular macromolecules on gel filtration in chloroform-methanol. Optical rotatory dispersion studies show the protein components have 50% α-helicity which agrees well with recent estimates of the α-helix content of whole membranes. However, viscosity studies indicate that the membrane proteins cannot exist as impermeable spheres which is the form of water-soluble globular proteins.