The HSPs (heat-shock proteins) of the 70-kDa family, the constitutively expressed HSC70 (cognate 70-kDa heat-shock protein) and the stress-inducible HSP70 (stress-inducible 70-kDa heat-shock protein), have been reported to be actively secreted by various cell types. The mechanisms of the release of these HSPs are obscure, since they possess no consensus secretory signal sequence. We showed that baby hamster kidney (BHK-21) cells released HSP70 and HSC70 in a serum-free medium and that this process was the result of an active secretion of HSPs rather than the non-specific release of the proteins due to cell death. It was found that the secretion of HSP70 and HSC70 is independent of de novo protein synthesis. BFA (Brefeldin A) did not inhibit the basal secretion of HSPs, indicating that the secretion of HSP70 and HSC70 from cells occurs by a non-classical pathway. Exosomes did not contribute to the secretion of HSP70 and HSC70 by cells. MBC (methyl-β-cyclodextrin), a substance that disrupts the lipid raft organization, considerably reduced the secretion of both HSPs, indicating that lipid rafts are involved in the secretion of HSP70 and HSC70 by BHK-21 cells. The results suggest that HSP70 and HSC70 are actively secreted by BHK-21 cells in a serum-free medium through a non-classical pathway in which lipid rafts play an important role.