Ultrafiltration experiments for the optical resolution of racemic phenylalanine were investigated in a DNA solution system and by using immobilized DNA membranes. D-Phenylalanine preferentially existed in the totally collected permeate solution in the ultrafiltration of DNA and racemic phenylalanine solution (i.e. DNA solution system), although the separation factor ranged from more than unity or less than unity depending on the permeation time when the concentration of DNA was less than 0.5ppm. This is explained by the fact that DNA sometimes releases phenylalanine depending on the permeation time in the dilute DNA solution (i.e. 0.01-0.5ppm) due to the conformational change of DNA as a function of time. On the other hand, L-phenylalanine preferentially existed in the permeate solution and D-phenylalanine preferentially existed in the concentrate solution in the ultrafiltration of racemic phenylalanine through the immobilized DNA membranes. This indicates that L-phenylalanine preferentially enters into the pores of the immobilized DNA membranes and permeates through the membranes due to the interaction between DNA and L-phenylalanine. The immobilized DNA membranes were categorized as channel type membranes.