Modulation of the host immune system represents a promising therapeutic approach against cancer, including multiple myeloma. Recent findings indicate that the NK group 2D (NKG2D)-and DNAX accessory molecule-1 (DNAM-1)-activating receptors play a prominent role in tumor recognition and elimination by cytotoxic lymphocytes, suggesting that the levels of NKG2D and DNAM-1 ligand expression on tumor cells may be a critical factor to improve the immune response against cancer. In this study, we tested the effect of 17-allylaminogeldanamycin and radicicol, drugs targeting the heat shock protein-90 (HSP-90) chaperone protein and displaying antimyeloma activity, on the expression of NKG2D and DNAM-1 ligands in human myeloma cell lines. We demonstrate that HSP-90 inhibitors are able to up-regulate both MHC class I chain-related (MIC) A and MICB protein surface and mRNA expression in human myeloma cell lines, without any significant effect on the basal expression of the DNAM-1 ligand poliovirus receptor CD155, or induction of nectin-2 and UL16-binding proteins. Activation of the transcription factor heat shock factor-1 by HSP-90 inhibitors is essential for the up-regulation of MICA/MICB expression and knockdown of heat shock factor-1 using small hairpin RNA interference blocks this effect. Moreover, in vitro and in vivo binding of heat shock factor-1 to MICA and MICB promoters indicates that it may enhance NKG2D ligand expression at the transcriptional level. Finally, exposure to HSP-90 inhibitors renders myeloma cells more efficient to activate NK cell degranulation and a blocking Ab specific for NKG2D significantly reduces this effect. Thus, these results provide evidence that targeting NKG2D ligands expression may be an additional mechanism supporting the antimyeloma activity of HSP-90 inhibitors and suggest their possible immunotherapeutic value.