SUMMARY.A simple immunoturbidimetric method for the measurement of beta-2 microglobulin in serum on a centrifugal analyser is described. The method detects beta-2 microglobulin in the range 0·4-16 mglL with an estimated within-batch and between-batch coefficient of variation of 5-8·5% and 9%, respectively. The method correlated well with two ELISA methods.Beta-2 microglobulin (B2M) is a protein of relative molecular mass II 800 found in low concentrations in serum, urine and other body fluids." 2 It has been shown to constitute the common chain of class I MHC molecules found on the surface of all nucleated cellsr': 4 Free B2M in the serum is filtered at the glomerulus and almost completely reabsorbed and catabolised by the proximal renal tubule cells." 6 The serum level is inversely related to the glomerular filtration rate 7 and has been shown to be a reliable early indicator of renal glomerular disease. II Urinary B2M has been shown to indicate renal tubular damage, but as the assay is seriously affected by urinary pH doubt must be cast on its reliability.Elevations of serum B2M are found, in the presence or absence of renal dysfunction, in inflammatory conditions, haematological and lymphoid malignancies, and a variety of nonmalignant conditions including AIDS.9 II This is thought to be due to increased cell turnover. rather than cell death. 12. 13 Its value as a tumour marker is limited by this lack of specificity.In multiple myeloma, however. it has been shown to be related to stage. tumour load and prognosis. A recent trial showed that the serum B2M was the single most important prognostic factor in the disease. 14