Beta-2-microglobulin (beta-2-MG) is a small membrane protein (11,800 Dalton) associated with the heavy chains of class I major histocompatibility complex proteins. It is expressed on the surface of almost all nucleated cells.

Serum beta-2-MG levels are elevated in diseases associated with increased cell turnover. Levels are also elevated in several benign conditions such as chronic inflammation, liver disease, renal dysfunction, some acute viral infections, and a number of malignancies, especially hematologic malignancies associated with the B-lymphocyte lineage.