Poly(L-lysine) (PL) is chemically modified on the basis of the commonly essential structures of effective gene carriers, which we previously found, in order to improve the insufficient gene-transferring efficacy of FL. The PL-derivatives were then subjected to gene transfer with the osmotic shock procedure. Poly(N-epsilon-trimethyl-L-lysine-co-serine) (PtmLS) showed much higher transient gene expression than PL with low cytotoxicity while poly(N-epsilon-trimethyl-L-lysine) and poly(l-lysine-co-serine) did not. These results strongly support the importance of both of the nonionic hydrophilic groups and quaternary ammonium groups of the gene carriers.