Bone sialoprotein (BSP) is a glycoprotein essentially found in mineralizing connective tissues. We have recently demonstrated that BSP is ectopically expressed by carcinomas that metastasize to bone with high frequency. Multiple myeloma (MM) is characterized by the localization of tumour plasma cells in the bone marrow. In this study, BSP expression was evaluated in human myeloma cell lines and in bone marrow aspirates and one ascites fluid from MM patients. BSP was detectable in conditioned media of MM cell lines. Using FACS analysis and in situ hybridization, we demonstrated that tumour cells from all MM patients and cell lines analysed express BSP at both the protein and the mRNA level.