The properties of the filter cake formed on the retentive ultrafiltration membranes in downward dead-end ultrafiltration of binary protein mixtures have been studied in a batchwise filter which has a sudden reduction in its filtration area. Both the average porosity epsilon(av) and the average specific filtration resistance alpha(av) of the filter cake were determined accurately on the basis of measurements of the variation of the filtrate volume with time. It was highlighted through this study that the electrical nature of macromolecules plays a significant role in determining the filtration flux rate in ultrafiltration of binary protein mixtures. In the pH range where both protein molecules are electropositive, the filter cake becomes loose and wet due to the electrostatic repulsive force acting between both proteins, leading to a higher filtration rate. In contrast, in the pH range where two protein molecules have opposite electrical charges, the filter cake becomes compact and dry because of the attractive force associated with oppositely charged solutes. It was, therefore, found that the average porosity reached a distinct minimum at a specified mixing ratio of two proteins in the case of oppositely charged proteins.