The low molecular weight protein beta2-microglobulin can be isolated from urine and other biological fluids. There is structural homology of beta2-microglobulin with the immunoglobulins, in particular to the CH3 domaine of the IgG molecule. Beta2-microglobulin can be detected on the cell membrane of most cell types and is associated with the histocompatibility antigens in a non-covalent binding. The immunological function of beta2-microglobulin is unclear. Antibody against beta2-microglobulin inhibits the mitogen- and antigen induced lymphocyte proliferation as well as responder and stimulator cell function in the mixed lymphocyte culture. The lack of structural polymorphism of beta2-microglobulin argues against a direct receptor function. It is more likely that beta2-microglobulin exerts a receptor-coordinating effect on the various receptor systems.