Serum beta 2 microglobulin (B2M) has recently been shown to be a powerful, although nonspecific, marker of multiple myeloma (MM) disease activity. Since the nature of cells producing high levels of B2M remains obscure in MM, we measured (in vitro) the spontaneous secretion of free B2M in 58 culture samples from 52 patients with MM. In comparison with samples from normal individuals and from individuals with monoclonal gammopathy of unknown significance, an abnormal secretion of B2M was found in 83% of samples containing myeloma cells. Levels of secretion significantly correlated with the percentage of tumor cells, tumor progression, and moreover, with the immunoglobulin type of the tumor. The highest levels of secretion were noted in IgG and IgA MM. Our present results would favor the hypothesis of a direct secretion of B2M by myeloma cells and emphasize the interest of B2M as a marker in a majority of (but not all) patients with MM.