Beta 2-microglobulin (B2m) was measured on serum samples in 274 patients with acute and chronic lymphoproliferative disorders (85 non-Hodgkin lymphomas-NHL, 30 Hodgkin lymphomas-HL, 34 B-cell chronic lymphocytic leukemias-B-CLL, 8 Waldenström macroglobulinemias-WM, 76 multiple myelomas-MM, 31 acute lymphoblastic leukemias-ALL, 10 hairy cell leukemias-HCL). Two hundred and four patients were studied at the time of diagnosis, and results were correlated to clinical stage, and histologic subtype in NHL, immunoglobulin type in MM, and immunologic phenotype in ALL. Moreover, B2m was tested during and after chemo- and/or radiotherapy, and results were correlated to response, progression or relapse. Elevated pretreatment B2m values were found in widespread forms of NHL and HL, in patients with B symptoms and in the unfavorable histologic subgroups of NHL. Rapid falls in levels followed therapy institution. In B-CLL and in MM a close relationship between B2m and cell mass was found. A significant B2m level reduction followed treatment, whereas its increase could detect a relapse. In ALL, serum B2m was only slightly above the normal range. B2m seems to reflect the total burden of malignant cells mainly in MM and B-CLL; in other lymphoproliferative disorders it provides less prognostic information.