Cation exchange membranes were prepared with 2,3-epithiopropyl methacrylate (ETMA)-2-acrylamide-2-methyl propane sulfonic acid (AMPS) copolymers. Transport of amino acids such as glycine (H2NCH2COOH), phenylalanine [C6H5CH2CH(NH2)COOH], and lycine [H2N(CH2)4CH (NH2)COOH] through the membranes against their concentration gradient was investigated by using the system containing the amino acid-HCl on one side and the amino acid on the other side of the membrane under various conditions. The ETMA-AMPS copolymer membranes transported amino acids against their concentration gradient and the transport of amino acids depended on both the structure of the amino acids and the composition of the membranes. The transport rate was greatly affected by the H+ concentration at the amino acid-HCl side of the membrane. The transport mechanism is also discussed.