Poly(L-lysine) was reacted with various azo-reagents, including p-phenylazobenzoic acid, pphenylazobenzoyl chloride, and p-phenylazobenzoic N-hydroxy-succinimide ester, to give polypeptides containing 5-44 mol % azobenzene units in the side chains. The conformation of the azo-modified polypeptides was investigated in connection with their photochromic behavior caused by the trans + CIS photoisomerization of the azo groups present in the side chains. In methanol/water solvent mixture, the 20% azo-poly(L-lysine) adopts the a-helix conformation. The helix stability was found to be higher when the azo side chains are in CIS than when they are in trans configuration. So irradiation a t 340 nm (trans-to-cw isomerization), and alternately a t 450 nm (cw-to-trans isomerization), produced reversible variations of the a-helix content. In hexafluoro-2-propanol/water/sodium dodecyl sulfate mixture, the 43% azo-poly(L-lysine) adopts a ,&structure, as indicated by CD spectra. Irradiation at 340 nm caused the disruption of the /I-structure and promoted the a-helix conformation. The effect was reversed upon irradiation a t 450 nm. The photoinduced + helix change was explained on the basis of the different geometry and hydrophobic character of the trans and the czs azobenzene units.