The strong acid/weak base amphoteric ion exchange fibers with different ratios of acidic and basic groups and different grafting percentages were prepared by grafting styrene and 4-vinylpyridine onto polypropylene fiber and then sulfonating with chlorosulphonic acid, under various reaction conditions, such as the feed ratio of styrene and 4-vinylpyridine, monomers concentration, sulfonation temperature, and sulfonation time, swelling time, etc. The produced strong acidic and basic groups are the sulfonic group and the pyridyl group, respectively. Only one sulfonic group is induced into an aromatic ring in most of PP-g-St-4VP and it is located in the para-position of styrene. The contents of the strong-acid group and the weak-base group of the PP-g-SO3-H-Py fibers produced in this work are in the range of 1.98 to 4.22 and 2.33 to 4.24 mmol/g, respectively. The strong acid/weak based amphoteric fibers obtained possess good static adsorption property for basic and acidic amino acids, and they also have a good dynamic adsorption and elution property for amino acids like related strong acid ion exchange fibers. (C) 2004 Wiley Periodicals, Inc. J AppI Polym Sci 92: 418-425, 2004.