A theoretical method is developed for calculation of melting curves of covalent complexes of DNA with antitumor drugs. The method takes into account all the types of chemical modifications of the double helix caused by platinum compounds and DNA alkylating agents: i) monofunctional adducts bound to one nucleotide; ii) intrastrand cross-links which appear due to bidentate binding of a drug molecule to two nucleotides that are included into the same DNA strand; iii) interstrand cross-links caused by bidentate binding of a molecule to two nucleotides of different strands. The developed calculation method takes into account the following double helix alterations at sites of chemical modifications: i) a change in stability of chemically modified base pairs and neighboring ones, that is caused by all the types of chemical modifications; ii) a change in the energy of boundaries between helical and melted regions at sites of chemical modification (local alteration of the factor of cooperativity of DNA melting), that is caused by all the types of chemical modifications, too; iii) a change in the loop entropy factor of melted regions that include interstrand cross-links; iv) the prohibition of divergence of DNA strands in completely melted DNA molecules, which is caused by interstrand cross-links only. General equations are derived, and three calculation methods are proposed to calculate DNA melting curves and the parameters that characterize the helix-coil transition.