A monoclonal antibody was used for the localization of the nuclear T3 receptor in different tissues of the adult rat: the liver, kidney, heart, lung, spleen, testis, and pituitary. In the liver, the immunoreactivity was found uniformly distributed in the nuclei of hepatocytes.Sections incubated with a control ascitic fluid or with the same ascitic fluid pre-adsorbed with purified receptor showed no specific staining. In the kidney, the immunoreactivity was higher in the epithelial cell of the proximal convoluted tubes and juxtaglomerular cells. In the heart, only the myocardial cells were stained. In the lung, the immunoreactivity was confined to type II pneumocytes and alveolar macrophages. In the spleen, only a few mature lymphocyte and macrophage cell nuclei were stained. These results show that: 1) the abundance of the nuclear T3 correlates with previous studies using hormone binding techniques; 2) the nuclear T3 receptor is selectively located in certain cell types, which possess a precise local function.Thyroid hormones are thought to elicit their biol¬ ogical activities through interaction with a nuclear receptor (Oppenheimer 1983; Oppenheimer et al. 1987). Much information has been obtained on re¬ ceptor structure and function using radiolabelled hormone of high specific activity. Antibodies pro¬ vide an additional tool with which to study recep¬ tors either bound or unbound to their ligand in a variety of physiological conditions. The produc¬ tion of polyclonal, and more recently, monoclonal antibodies against the 3,5,3'-triiodothyronine (T3) receptor using rat liver as a source of antigen have been documented (Faure et al. 1986;Luo et al. 1988). We have shown by means of protein blotting techniques (Luo et al. 1988) that the tissue distribu¬ tion profile of the receptor is similar to the ones re¬ ported in previous results obtained by hormone saturation analysis (Oppenheimer et al. 1974).These studies, however, do not document the cell type distribution of the T3 receptor as this infor¬ mation is clearly important to understand the basis of the biological action of thyroid hormones. We have therefore utilized immunocytochemical techniques on frozen tissue sections to study the cellular localization of the receptor in liver, kidney, heart, lung and spleen of the adult rat. Materials and Methods MaterialsSprague-Dawley male rats (150-200 g) were obtained from a commercial supplier (Canadian Breeding Farms, St. Constant-Laprairie, Canada). The animals were housed in attenuated sound and temperature, light con¬ trolled room, and allowed free access to rat faboratory chow and water. The anti-mouse IgG, peroxidase antiperoxidase (PAP) compiexes and anti-mouse IgG conjugated with fluorescein isothiocyanate (FITC) were kindly provided by Dr G. Pelletier and Dr J. P. Valet (C.H.U.L., Québec), 3,3'-diaminobenzidine tetrahydrochloride (DAB) and anti-foam A emulsion were pur¬ chased from Sigma (St. Louis, MO).