In protein ultratiltration, solution environment factors such as pH and solvent density are important in controlling the filtration flux. Also, the properties of the filter cake of macromolecular solutes formed on the membrane surface play a vital role in determining the filtration flux in ultratiltration. On the basis of the sedimentation velocity technique and the high-speed sedimentation equilibrium technique using an analytical ultracentrifuge, the influence of the protein solution pH on the time variation of the filtration flux in unstirred dead-end ultrafiltration was accurately described. It was clearly demonstrated that the low filtration flux around the isoelectric point is mainly due to the build-up of compact filter cake, which causes a large hydraulic flow resistance. Furthermore, it was shown that the solvent density ρ has a large effect on the dynamically balanced filtration rate in upward ultrafiltration just as it does on the sedimentation coefficient in ultracentrifugation.