In order to prepare gene carriers responsive to an endosomal pH, amino groups of linear poly(allylamine) (PAA) or poly(L-lysine) (PLL) were coupled with a-carboxyl groups of α-amino acids (Gly, His, Lys, Arg, and Orn). Acid-base titration indicated that Lys-, Arg-, and Orn-pendant polymers had both strongly basic groups and endosomal pH-responsive α-amino groups. These polymers, like PAA and PLL, formed stable complexes with DNA. Lys-, Arg-, and Orn-pendant polymers were effective transfection reagents independent of the backbone polymers. The pH-responsive α-amino groups enhanced transfection activity as shown by the observation that acetylation of the α-amino group resulted in a considerable loss in transfection activity. These results strongly suggested a lysosomotropic activity of the α-amino groups. Among the α-amino acid-pendant polymers tested, the Orn-pendant polymer exhibited the highest transfection activity/toxicity index. Since PLL with α-amino acid-pendants is composed of naturally occurring amino acids, it is expected to be biodegradable, and these reagents have promise as gene carriers.