β2-microglobulin (β2M) levels were measured in 217 Edinburgh drug users to assess their usefulness as a marker for HIV-related disease. Eighty HIV-seronegative drug injectors had significantly higher levels than 100 HIV-seronegative blood-donor controls. Amongst 137 asymptomatic HIV-seropositive drug users, those who were defined as continued drug users had significantly higher β2-M levels and percentages of CD3+ T lymphocytes with DR Class II expression than non-injecting drug users. β2M levels correlated with the percentage of activated DR + CD3 + T lymphocytes. These findings indicate that changes in β2M levels may reflect differences in drug-injecting behaviour and are not influenced solely by HIV status or progression. These changes in β2M probably represent differing degrees of immunostimulation resulting from the antigenic challenges afforded by continued or frequent drug injection. It is important to establish normal ranges for β2M from HIV-seronegative controls who are matched with respect to risk group and behaviour. All these factors should be taken into account if β2M is to be used as a marker of HIV progression.