Thermodynamic parameters of DNA melting in the presence of a peptide bronchogen in various concentrations were estimated on a differential scanning microcalorimeter. Bronchogen was shown to serve as a DNA-stabilizing agent. Bronchogen increased the melting temperature of DNA from calf thymus and mouse liver by 3.1°C in a narrow range of r (molar ratio of bronchogen/DNA b.p., 0.01-0.055). A further increase in r was not accompanied by changes in the melting temperature. The complex melting enthalpy (ΔH(melt)) remained unchanged in this range of r (0.01-1.0). ΔH(melt) for DNA from the thymus and mouse liver was 11.4 and 12.7 cal/g, respectively. Our results indicate that bronchogen is not an adenine-thymine-specific or guanine-cytosine-specific ligand. The type of binding is considered as strong and occasional. The binding occurs with both strands of DNA (mainly with nitrogen bases).