In order to determine whether the immunological abnormalities described in intravenous drug addicts (IDA), are due to HIV infection, other viral infections or to the abuse of narcotic drugs, we studied the T lymphocyte subsets and serological markers of infection with hepatitis B and delta virus, cytomegalovirus and Epstein Barr virus, in 49 IDA. The immunological and serological features of IDA were compared with the control group, made up of 20 healthy subjects. In intravenous drug abusers we found a significant increase in the number of total lymphocytes (P less than 0.01), T-lymphocytes (P less than 0.05), T-suppressor cells (P less than 0.05), and serum IgG levels (P less than 0.0001) as compared with the control group. The prevalence of serological markers of infection with hepatitis B virus, hepatitis delta virus, cytomegalovirus and Epstein Barr virus was significantly higher in IDA as compared with the controls. In conclusion our study demonstrates that T-lymphocyte subsets in IDA seronegative for HIV infection are characterized by an enhancement of peripheral lymphocyte cells with a normal OKT4/OKT8 ratio.