Beta-2-microglobulin (B2M) forms the small invariable light chain subunit of class I HLA antigens on the cell membrane of all nucleated cells. During the continuous turnover of the HLA molecules, B2M is shed from the cell membrane into blood. Lymphocytes are the main source of serum free B2M. Serum B2M concentration is increased in renal diseases, various malignant diseases and some inflammatory and autoimmune disorders. In lymphatic malignancies serum B2M has significant prognostic value. Interferons (IFNs) have the ability to enhance the expression of class I and II histocompatibility antigens. Accordingly, IFNs cause a rise in formation and release of B2M. Currently, treatment with IFN alpha is used in diseases, like multiple myeloma, where serum B2M measurements are used to assess tumor burden. We have measured serum B2M levels during IFN alpha treatment in patients with both multiple myeloma and chronic myeloproliferative diseases, and IFN alpha caused a significant increase in serum B2M. It can be concluded that use of IFN alpha abolishes the value of serum B2M as an indicator of disease activity.