Beta-2-microglobulin (beta 2M) is a low-molecular protein which is filtered freely over the glomeruli. Under normal circumstances, more than 99.9% is resorbed in the proximal tubuli of the kidneys and is metabolized there. In renal disease with damage to this segment of the nephron, eg acute tubulo-interstitial nephropathy, increased quantities of beta 2M are excreted in the urine. If the rate of glomerular filtration is reduced, serum-beta 2M is increased and this is also the case in persons with increased cell division despite normal renal function. Serum-beta 2M is, therefore, raised in numerous malignant diseases and reflects the size of the tumour mass. During cytostatic treatment of myelomatosis and chronic lymphatic leukaemia, the serum-beta 2M levels decrease on remission and increase on relapse. In acute leukaemia and malignant lymphoma with infiltration of the CNS, similar conditions prevail for CSF-beta 2M. beta 2M is a sensitive but non-specific parameter in inflammation and infection. Determination of beta 2M in saliva has been found to be of value in Sjögren's syndrome. In HIV-positive patients, a high serum-beta 2M level is a poor prognostic sign. Determinations of CSF-beta 2M in AIDS patients must be presumed to be of value not only in the diagnosis of cerebral AIDS and other (latent) CNS infections but also to demonstrate complicating cerebral malignant lymphomata at an early stage.