Polyethyleneimine derivatives of adenine with L- and D-serine as spacers were prepared. The polymer having D-serine spacer 2 gave a stable polymer complex with poly(uridylic acid) (poly U), but the polymer having L-serine spacer 1 gave an unstable polymer complex. The reason was concluded to be caused by the steric repulsion of L-serine units along the polymer chain when the polymer formed polymer complex with poly U.