Quantitative analysis of beta-2-microglobulin (beta-2-M) may be a useful test for early diagnosis of acquired immuno-deficiency syndrome (AIDS). Even if beta-2-M is an unspecific marker, 96% of 45 patients with confirmed AIDS and 82% of 45 patients with lymphadenopathy have excessive (3 mg/l) beta-2-M value. beta-2-M is also useful in epidemiological studies, especially in subjects in close contact with patients, and in polytransfused hemophiliacs. At last, beta-2-M may be useful as a screening method in blood banks: from a study of 929 sera, excessive beta-2-M value was observed in 16 cases, which were excluded as blood donors.