Properties of the stoichiometric complex formed by poly(L-lysine) cations and dodecyl sulfate anions in organic solvents and in the solid state were investigated via viscometry, H-1 NMR, infrared, circular dichroism, and X-ray diffraction techniques. In dilute solutions in chloroform with up to 10 vol % of trifluoroacetic acid, the complex neither forms interchain aggregates nor dissociates to yield free surfactant. In chloroform solutions containing small amounts of trifluoroacetic acid (1-2 vol %), the polypeptide chains are in an alpha-helical conformation, while a transition to a disordered form occurs at higher trifluoroacetic acid contents (4-6 vol %). The helix-coil transition of the poly(L-lysine) chains is accompanied by a decrease in the H-1 spin-lattice relaxation times of the polypeptide chain segments, while the relaxation times of the surfactant chains remain essentially unchanged. In chloroform-trifluoroacetic acid mixtures, the alpha-helical conformation of the poly(L-lysine) chains is stabilized by increasing temperature. Polypeptide chains in the solid complex can adopt either alpha-helical or beta-sheet conformations, depending on the trifluoroacetic acid content of the chloroform solution used for film casting. The solid complex is organized into a lamellar structure consisting of alternating layers of poly(L-lysine) chains and double layers of surfactant, arranged tail to tail.