In order to obtain a thermo-responsive polymer which exhibits hydration change in response to foreign optically active compounds as external stimuli, binary copolymers composed of N-(S)-sec-butylacrylamide ((S)-sec-BAAm) and N-isopropylacrylamide (IPAAm) were synthesized by ordinary radical copolymerization. Turbidity of the polymer aqueous solutions containing various concentrations of tryptophan (Trp) isomer was measured to determine lower critical solution temperature (LCST) values. Poly((S)-sec-BAAm-co-IPAAm) having 50 mol% of (S)-sec-BAAm content was a water-soluble polymer which exhibited LCST at 23.1 degrees C. The LCST of the copolymer aqueous solution in the presence of L-Trp was shifted remarkably to 34.5 degrees C, while that in the presence of D-Trp was 28,7 degrees C, On the other hand, a racemic (R,S)-sec-BAAm-containing copolymer showed the same phase transition temperatures in the presence of D- and L-Trp. There was no influence of Trp concentration on LCST for poly(N-isopropylacrylamide) aqueous solution. It was clear that the optically active (S)-sec-BAAm in the copolymer stereospecifically interacted with L-Trp, showing a relatively higher LCST value. The LCSTs of the copolymer increased in response to increasing concentration of L-Trp, It was found that the introduction of the optically active moieties into the copolymer chains resulted in the phase-transition behavior changes of the copolymers in response to the aromatic amino acids in aqueous solution. The results, therefore, revealed that the thermo-responsive copolymer with optically active moieties demonstrated chiral recognition to the enantiomers.