AbstractWater‐soluble poly‐L‐lysine‐ and poly‐DL‐lysine‐copper(II) complexes were prepared in aqueous alkaline solutions.The complex‐formations of poly‐L‐lysine and poly‐DL‐lysine with cupric ions in aqueous solutions were studied with pH titration and electronic spectroscopy and measurements of optical rotatory dispersion, circular dichroism and solution viscosity. From the results it was revealed that the poly‐L‐lysine forms two kinds of complexes, being formed on the degree of neutralization α0.5, respectively. The former complex consists of a cupric ion coordinated with four amino nitrogen atoms of the side chains. Some extents of helical conformation of poly‐L‐lysine are held after the complex formation with cupric ions at a pH over 10. The results on the poly‐L‐lysine‐copper(II) complexes were compared with those on t