Beta 2-microglobulin level has been evaluated in 183 male intravenous drug users and 50 apparently healthy blood donors. The aim of the study was to assess the possible association between high values of the peptide and both HIV infection and disease progression in intravenous drug users. Beta 2-microglobulin values were found significantly higher in AIDS patients and other symptomatic subjects than in asymptomatic, both seropositive and seronegative, intravenous drug users. Among asymptomatic drug users, no significant difference in the values was found.