The heat shock protein 70 (Hsp70) family members function as ATP-dependent molecular chaperones that assist in the folding of newly synthesized polypeptides and in the refolding of misfolded/aggregated proteins. These heat shock proteins comprise at least eight sets of molecular groups that share high homology, but differ from each other in their expression level and subcellular localization. Hsp72, which is also known as Hsp70 and Hsp70-1, is localized mainly in the cytoplasm but is also found in the nucleus. Stress-induced Hsp72 functions as a chaperone enabling the cells to cope with harmful aggregations of denatured proteins during and following stress. The difference in the function of Hsp72 from that of other Hsp70 members, however, remains unclear. We report the establishment of a monoclonal antibody specific for Hsp72 using the rat medial iliac lymph node method. Immunoblot analysis revealed that our monoclonal antibody against Hsp72 specifically identified the 65 kDa protein. Immunocytochemical staining also revealed that Hsp72 localized in the cytoplasm and nucleus, and aggregated in the nucleus in response to heat stress. This MAb against Hsp72 will allow for further studies to elucidate the mechanism by which Hsp72 is localized in the cell in response to stress stimuli, and aid in the identification of specific interacting molecules.