Nutrients usually cross the outer membrane of Escherichia coli by diffusion through water-filled channels surrounded by a specific class of protein, porins. In this study, the rates of diffusion of hydrophilic nonelectrolytes, mostly sugars and sugar alcohols, through the porin channels were determined in two systems, (a) vesicles reconstituted from phospholipids and purified porin and (b) intact cells of mutant strains that produce many fewer porin molecules than wild-type strains. The diffusion rates were strongly affected by the size of the solute, even when the size was well within the "exclusion limit" of the channel. In both systems, hexoses and hexose disaccharides diffused through the channel at rates 50-80% and 2-4%, respectively, of that of a pentose, arabinose. Application of the Renkin equation to these data led to the estimate that the pore radius is approximately 0.6 nm, if the pore is assumed to be a hollow cylinder. The results of the study also show that the permeability of the outer membrane of the wild-type E. coli cell to glucose and lactose can be explained by the presence of porin channels, that a significant fraction of these channels must be functional or "open" under our conditions of growth, and that even 10(5) channels per cell could become limiting when E. coli tries to grow at a maximal rate on low concentrations of slowly penetrating solutes, such as disaccharides.