The sedimentation rate in ultracentrifugation of an aqueous solution of the mixtures of bovine serum albumin (BSA) and egg white lysozyme has been measured using Schlieren optics＇ in an analytical ultracentrifuge. It was highlighted through this study that the electrical nature of macromolecules plays a significant role in determining the sedimentation behavior in ultracentrifugation of binary protein mixtures. In the pH range where both protein molecules were electropositive, the molecules sedimented independently due to the electrostatic repulsive force acting between BSA and lysozyme molecules. In contrast, in the pH range where two protein molecules had opposite electrical charges, the complex was formed because of the electrostatic attraction between BSA and lysozyme molecules, and it sedimented with a sedimentation rate larger than that for free BSA alone. However, when enough salts were present in the solution, the electrostatic attraction between solutes was reduced due to repression of the electrical double layer, and thus a complex with a large sedimentation rate did not form. This study revealed that the solution environment can have profound effects upon the sedimentation behavior in ultracentrifugation of binary protein mixtures.