A novel 40-kDa heat-shock protein hsp 40 in mammalian cells has been recently identified to be a homolog of bacterial DnaJ protein. We have previously shown the colocalization of hsc70 (p73, constitutive form) with hsp40 in the nucleoli of heat-shocked HeLa cells. In this report we further investigated intracellular translocation and localization of hsp40 and hsp70 (both constitutive p73 and inducible p72) in several mammalian cells. Translocation kinetics of hsp40 during heating at mild temperature were almost the same as those of hsp70 in HeLa cells. Hsp40 colocalized not only with hsc70 (p73) but also with hsp70 (p72) in heat-shocked HeLa (human), HA-1 (Chinese hamster) and NRK (rat) cells. Direct interaction of hsp40 with hsp70 (p73 and/or p72) was observed in all cells tested by immunoprecipitation methods. Also, treatments of cells with cytoskeleton- acting drugs such as cytochalasin E, colchicine and taxol had no effect on the heat-induced translocation of hsc70 (p73) and hsp40 in NRK cells. These results strongly suggest that hsp40 and hsp70 (p73/p72) form a complex in the cytoplasm at normal temperature, translocate together and colocalize in the nuclei and nucleoli upon heat-shock, and that they may function cooperatively to repair (ref old) denatured proteins under stress conditions.