It has been suggested that members of HSP families represent the surface target of immune responses leading to tumor rejection in mice. Here we report that tumor cells, compared with normal cells, constitutively expressed 2- to 10-fold higher levels of intracellular HSP90. Moreover, in the absence of environmental stress, 2 lines (out of 6) expressed the "inducible" HSP72, which was also detectable in fresh tumor cells. HSP72 expression was not regulated during the cell cycle, in contrast with what has been observed with normal cells. Both HSP90 and HSP72 proteins exhibited a heterogeneous pattern of intracellular distribution in most cells, HSP72 being confined mainly to the nuclear compartment. Finally, we could detect both HSP90 and, to a lesser extent, HSP72 (that are generally believed to be located intracellularly) at the surface of some tumor cell lines. We conclude that tumor cells differ from normal cells in their pattern of HSP expression; this might imply a role of HSPs in eliciting an immune response against cancer.