Beta2-microglobin (ß2M) is a low molecular weight (11.8 kD) protein present on the surface of many cells. Beta2-microglobin exists in two forms: in free form or membrane form, identified as the light chain of the class I major histocompatibility complex (MHC). Despite its small size, allowing it to pass through the glomerular membrane, less than 1% of the filtered beta2-microglobulin is found on urinalysis, the rest being reabsorbed and metabolised in the proximal tubule of the kidneys. The assay of urinary beta2-microglobulin mainly enables kidney disease to be detected, revealing a tubular disorder of various origins (infectious, drug, congenital, etc.) or subsequent to a kidney transplant.