To gain a basic understanding of the ultrafiltration characteristics of suspensions containing both fine particles and macromolecules, dead-end ultrafiltration of mixtures of rutile titanium dioxide (TiO(2)) and bovine serum albumin (BSA) was conducted under constant pressure, and the effects of pH and electrolyte concentration on the filtration properties were investigated from the standpoint of the interactions between particles and macromolecules. BSA molecules have a tendency to be adsorbed onto TiO(2) particles in the mixed suspension, which affects the stability of TiO(2) particles significantly. At pH conditions where TiO(2) and BSA carry opposite charges, BSA adsorption induces a marked change in the zeta potential of TiO(2), resulting in a dramatic change in the particle size. At pH conditions where TiO(2) and BSA carry the same electric charge, the particle size varies greatly upon addition of electrolyte due to its charge-shielding effect and the combined effect of electrolyte and BSA coexisting in liquid. Thus, it can be concluded that the ultrafiltration characteristics of particulate suspensions containing macromolecules are determined by the stability of particles.