Ternary systems consisting of a conformationally variable chain capable of assuming either a rodlike helix or a random coil conformation, a randomly coiled polymer, and a solvent were treated according to the lattice model proposed by Flory. The partition function has been derived in consideration of the Flory-Matheson expression related to the free energy contribution from the chain conformation changes. A marked increase of the miscible isotropic zone was predicted when the chain rigidity of the conformationally variable polymer in the isotropic phase is reduced. The conformationally variable chain tends to adopt a rodlike form as long as it enters the anisotropic phase in which the randomly coiled polymer is essentially excluded. The influence of various molecular structure parameters, solvent-polymer interaction, and temperature was also studied.