Ultrafiltration experiments for optical resolution of racemic phenylalanine and leucine were performed in a solution system containing bovine serum albumin (BSA) and by using immobilized BSA membranes. It was found that D-phenylalanine preferentially existed in the permeate at pH 7.0 due to the binding of BSA to L-phenylalanine and that the concentration ratio Of D-isomer to L-isomer in the permeate increased with a decrease in the feed concentration of the racemate in the solution system. This observation is explained by a site saturation mechanism. A minimum point was observed in the plot of the separation factor vs. feed concentration of leucine for the optical resolution of leucine in the solution system. It is suggested that BSA in the solution has two binding sites to leucine (i.e., binding sites to D-leucine and L-leucine). It was found that the immobilized BSA membranes efficiently demonstrated optical resolution of racemic amino acids, although the racemic amino acids were optically resolved less effectively in the ultrafiltration using the immobilized BSA membranes than in the BSA solution.