Microcalorimetry was used to measure the change in enthalpy for the scission of calf thymus DNA (ct-DNA) induced by adriamycin (ADM) in the presence of ferric ions, Vitamin C, and oxygen. At 298.15 K and pH 7.4, the overall molar reaction enthalpy for this cleavage was -147.1 kJ/mol, noticeably higher than that by the mixture of Fe3+, Vitamin C, and O-2 Under the same conditions, the enthalpy change for the damage of ct-DNA by the mixture of adriamycin, ferrous ions, and oxygen, however, was nearly zero, indicating that this mixture can not induce any detectable degradation of DNA. These results suggest that both the activated adriamycin and hydroxyl radical attack DNA strands during the cleavage. A possible mechanism for the cleavage of DNA induced by adriamycin is proposed based on the calorimetric measurements. A novel thermodynamic model for the interactions of DNA with small molecules is also suggested. This is a convenient method to calculate both the binding constant (K-b) and the standard thermodynamic parameters (Delta(b)H(m)(0), Delta(b)C(m)(0), and Delta(b)S(m)(0)) for the binding of adriamycin-Fe3+ complex to ct-DNA by the calorimetric data. This nucleotide binding reaction is driven by a favorable enthalpy change, with a large unfavorable entropy change. This result indicates that the binding results in structural changes accompanied by an increase in the order of the whole system, implying that an intercalation mode is involved in adriamycin-mediated breakage of DNA.