Employment of radiolabeled antibodies in biological studies allows their specific accumulation in organs and tissues to be detected. However, stability of such labeled antibodies depends on both the method of labeling and particular experimental conditions. Therefore, stability of labeled antibodies should be determined in every particular experiment. The present paper describes the results obtained in the study of the radiochemical purity and stability of the 125I-labeled conjugates of antibodies to the gliofibrillary acidic protein (GFAP), endothelial antigen AMVB1, and non-specific mouse IgG immediately after their synthesis and at various time points after intravenous administration to rats with experimental C6 glioma. Their stability was determined in the samples of blood, brain, liver, spleen, and kidney by precipitation with trichloroacetic acid. Electrophoretic analysis, thin-layer chromatography, and immunohistochemical tests have proved the radiochemical purity, immunochemical competence, and stability of the labeled antibodies in vivo.