Geometrical properties of the microstructure of whole bovine cervical mucus were studied. An experimental technique was developed for measuring the flow of fluid through the mucus microstructure in response to application or a prescribed external pressure gradient. The data obtained were analyzed in conjunction with a mathematical model of the hydrodynamics of the flow-permeation process. The sizes of typical interstices within the microstructure were calculated to be of the order of 1 micrometer, with typical macromolecular filament diameters being of the order of 100 A. These dimensions were interpreted as representative of an equivalent network giving rise to measured flow permeability. The values of filament size showed a strong experimental correlation with the solids content of the mucus.