l-Menthol was fixed on cellulose acetate membranes with pore diameters of 0.2, 0.45 and 0.8 mu m by the plasma polymerization method at various times of plasma irradiation. The fractional fixation of l-menthol increased with increasing time of plasma irradiation, resulting in a decreasing pure water permeability of the membrane. The optical resolution of racemic tryptophan, phenylalanine and tyrosine was carried out by ultrafiltration using the plasma polymerized membrane, which was prepared by polymerizing l-menthol onto a cellulose acetate membrane with 0.2 mu m membrane pore diameter exposed to plasma irradiation for 60 min. The membrane was permeable with respect to D-isomers of all three racemic amino acids in preference to L-isomers. The larger value of the separation factor was obtained at lower volume flux. The maximum value of the separation factor obtained for DL-Trp was 8 at Jv = 1.5 X 10(-8) m . s(-1).