Water-soluble cationic polymers, poly(histamine acrylamide)s (PHAs), with superior buffer capacity at the endosomal pH range were designed, prepared, and investigated for non-viral gene transfection. PHAs were obtained with molecular weights ranging from 9.2 to 28.7 kDa through controlled radical polymerization of histamine acrylamide (HA). Acid-base titration results displayed that all PHA polymers had a remarkably high buffer capacity of about 70% at pH 5.1-7.2. 12.7-28.7 kDa PHAs were able to effectively condense DNA into nano-sized (< 220 nm) polyplexes with moderate positive surface charges (+13-+19 mV) at N/P ratios >= 10/1. CCK assays indicated that polyplexes of 12.7 and 17.5 kDa PHAs were nontoxic to COS-7 cells up to a tested N/P ratio of 20/1. Interestingly, the in vitro transfection using pCMV-Luc and pEGFP-C1 plasmid DNA as reporter genes showed that polyplexes of 12.7 kDa PHA formed at an N/P ratio of 20/1 mediated efficient transfection in COS-7 cells under 10% serum conditions, with transfection efficiencies comparable to that of 25 kDa polyethylenimine control. Their versatile design of structures, controlled synthesis, low cytotoxicity, and high transfection activity render PHA-based cationic polymers particularly interesting for the development of safe and efficient non-viral gene delivery systems. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 3366-3373, 2011