We previously showed that HIV-1 protease inhibitors slowed the proliferation of human myeloid leukemia cells and enhanced their differentiation in the presence of all-trans retinoic acid (ATRA). In this study, we found that protease inhibitors, including ritonavir, saquinavir, and nelfinavir, but not indinavir, induced growth arrest and apoptosis of U266, RPMI8226, and ARH77 human multiple myeloma (MM) cells in association with down-regulation of antiapoptotic protein Mcl-1. Also, protease inhibitors inhibited the survival of freshly isolated MM cells from patients. In contrast, these protease inhibitors did not affect survival of normal B cells and colony formation of myeloid committed stem cells (CFU-GM) from healthy volunteers. In addition, we found that all of the protease inhibitors, except for indinavir, blocked interleukin-6 (IL-6)-stimulated phosphorylation of both signal transducer and activator of transcription 3 (STAT 3) and extracellular signal-regulated kinase 1/2 in U266 and RPMI8226 MM cells. Moreover, the protease inhibitors inhibited both the basal and IL-6-stimulated STAT 3/DNA binding activity in U266 cells as measured by an ELISA-based assay. Furthermore, ritonavir inhibited production of vascular endothelial growth factor one of the targets of STAT 3, in U266 and RPMI8226 cells as measured by ELISA. Taken together, protease inhibitors might be useful for treatment of individuals with MM.