Summaryβ2m is a low molecular weight protein of 100 amino-acids which is non-covalently associated with the heavy Chain of HLA-A, B and C antigens on the surface of neariy all cells, with the exception of erythrocytes. Its amino-acid sequence and three dimensional structure bear a striking similarity with domains of heavy and light chains of immunoglobulins and with HLA antigens.β2m is released from cell surface into biological fluids where it is found mostly as free monomer. Lymphocyte activation by mitogens, allogeneic cells or specific antigens, as well as leukocyte interferon administration, enhanceβ2m release. The kidney represents the major site ofβ2m catabolism.β2m is filtered through the glomeruli and serumβ2m levels are negatively correlated with the glomerular filtration rate. About 99 % of filteredβ2m is reabsorbed and catabolized by proximal tubular cells. Changes in serumβ2m concentrations can be due either to reduced glomerular filtration rate or to increased synthesis at it the case in malignant diseases, lymphoproliferative disorders or chronic inflammatory diseases (systemic lupus erythematosus, rheumatoid arthritis, Sjögren's syndrome, sarcoidosis, inflammatory bowel diseascs, hepatitis or cirrhosis). Increased urinary excretion reflects primarily defective tubular reabsorption and catabolism; it represents the hallmark of primary tubulo-interstitial disease.