Serum beta 2 microglobulin (beta 2-M) levels were determined in patients with Acquired Immunodeficiency Syndrome (AIDS), AIDS Related Complex (ARC), Persistent Generalized Lymphadenopathy (PGL), healthy intravenous drug addicts (IVDA) and heterosexual controls. Seventy-eight out of 79 AIDS patients (98.7%) exhibited elevated beta 2-M levels. High levels of beta 2-M were also found in 83 of 100 (83%) of PGL/ARC patients and in 24 of 56 (42.8%) healthy IVDA. Patients with AIDS had significantly higher mean beta 2-M levels when compared with all other groups. The mean levels of PGL/ARC patients were significantly higher than those of healthy IVDA and the mean levels for healthy IVDA significantly differ from those of the heterosexual controls. After 2-24 months of follow up three out of four PGL/ARC patients whose serum beta 2-M was greater than 8.0 mg/l developed AIDS.