Tumor-associated antibodies of human IgG1 subclass were eluted from cell-surface antigens of human carcinoma cells and studied by differential scanning calorimetry and binding to local conformational probes, protein A from Staphylococcus aureus and a monoclonal antibody targeted to the CH2 domain of the Fc fragment. At pH 2.0-7.0, we observed virtually identical enthalpies of thermal unfolding for IgG1 from normal human sera and tumor-associated IgG1. The exact values of calorimetric enthalpy (Delta h) at pH 7.0 were 6.1 and 6.2-6.3 cal/g for IgG1 from normal serum and IgG1 from carcinoma cells, respectively. The affinity constants of protein A binding to the CH2--CH3 domain interface demonstrated differences between serum IgG1 and tumor associated IgG1 that did not exceed 3-8-fold. The binding affinity toward the anti-CH2 monoclonal antibody determined for serum IgG1 and IgG1 from carcinoma cells differed not more than 2.5-fold. The thermodynamic parameters of IgG1 from carcinoma cells strongly suggest that protein conformational stability was essentially unaltered and that the Fc fragment of the tumor-derived IgG1 preserved its structural integrity.