Antigenic peptides have been found associated with heat shock proteins (HSP) including cytoplasmic HSP70 and heat shock cognate protein 70 as well as the endoplasmic reticulum-resident glucose-regulated protein 94. Recently, HSP70 transfection has been reported to increase MHC class I cell surface expression and antigen presentation on mouse melanoma B16 cells (Wells et al., Int. Immunol. 1998. 10: 609). To analyze the effect of HSP70 on MHC class I cell surface expression and lysability of target cells we transfected a human melanoma cell line with the rat Hsp70-1 gene using the Tet-On system for conditional overexpression of HSP70. Induction of HSP70 did not increase cell surface expression of HLA class I molecules in general or individual HLA-A and B antigens in particular. Nonetheless, induction of HSP70 enhanced susceptibility of these cells to lysis by allospecific CTL. The same effect was observed using an HLA-A2-restricted tyrosinase-specific CTL clone after pulsing the tyrosinase-negative target cells with the specific peptide. Thus, HSP70 induction can increase killing by CTL without affecting MHC class I cell surface expression or antigen processing. This effect of HSP70 appears to be different from the commonly found protection exerted by HSP70 against stress like heat shock, and might be mediated by improving CTL-induced apoptosis.