Novel amphoteric particles are prepared by an interfacial polyaddition reaction of ethylene glycol diglycidyl ether (EGDGE) and L-lysine in a water-in-oil (W/O) miniemulsion system. The zeta-potential of the copolymer particle as a function of pH indicates a sigmoid curve with an isoelectric point at pH 8.7 due to the existence of secondary amino and carboxyl groups in the copolymer chain. The obtained amphoteric particle bears positive charge under pH 8.7 and negative charge over pH 8.7. This specific property of the pH-responsive amphoteric particle is employed to separate single-stranded DNA (ssDNA). The trapped ssDNA at pH 7.0 is almost released by changing pH to 11.0, which can be carried out repeatedly. (c) 2005 Wiley Periodicals, Inc.