AbstractThe interaction of polypeptides with dyes in a lyotropic mesophase, consisting of Tween 80 (monooleate of the ether of sorbitan with 4 mol of polyethyleneglycol) and water, was studied, and the arrangement of dyes in the mesophase was discussed on the basis of the spectral change of circular dichroism under an electric field. The following results were observed. (1) The addition of a polypeptide to the mesophase considerably influenced the helical pitch of the mesophase. (2) In the mesophase the complexes of polypeptides with dyes were formed more easily than in aqueous solution. (3) When this mesophase was used as an orientation solvent under the electric field, the observed variation of circular dichroism bands could be used for an assignment of each band polarization. Thus, in the acidic region, acridine orange molecules are bound in such a manner that their long‐axes of molecular plane form a right‐handed super‐helix around the core of the α‐helix of poly(α,L‐glutamic acid). In the alkaline region, the poly(α,L‐glutamic acid)‐acridine orange complex cannot take a helical form, even when the mole ratio of polypeptide residues to dye