The influence of the extended interacted under adsorption ligands with a selective binding on the DNA helix-coil transition has been theoretically studied. It was found that contact interaction between ligands or/and their extent give rise to a marked non-linearity of the GC-content dependence of the melting temperature. This non-linearity causes a few features of the dependence of the melting range width on ligand concentration [delta T(C0)]. Such as a non-monotony of the delta T (C0) increase in the presence of ligands increasing the difference between the thermostabilities of poly(d(A-T)] and poly[d(G-C)] polymers. The degree of a non-linearity defines the character of changes of the form of the differential melting curves in the presence of ligands.