Cellular stress proteins like the classical heat-shock proteins (HSPs) hsp70 (1 –3 ), hsp90 (3 ) and hsp110 (4 ); the glucose-regulated proteins gp96/GRP94 (3 ,5 ) and grp170 (4 ); as well as the endoplasmic chaperone calreticulin (6 ,7 ) have been shown to induce cytotoxic T-cell responses and protective immunity when isolated from tumor or infected cells and used to vaccinate animals (8 –10 ; reviewed in refs. 11–17). The specificity of the immune responses is owing to antigenic peptides associated with the HSPs (18 –26 ). The HSP-peptide complexes are taken up by professional antigen-presenting cells (APCs) for effective presentation of the peptides to T cells (19 ,27 –33 ). The extensively studied endoplasmic reticulum-resident chaperone gp96 is most efficient and promising in this regard (34 –39 ); encouraging clinical studies with human cancer patients (40 ).