Serum levels of immunoglobulin classes and IgG subclasses were determined in healthy seronegative (n = 50) and in HIV-1 seropositive adults (n = 50) from Tanzania, a region with heterosexual HIV-1 transmission pattern. The mean levels of total IgG, IgG1, IgG3, IgA, IgM, and IgD obtained were found to be significantly higher in HIV-1 seropositive sera than in control sera. The mean IgA, IgM, IgD and IgG3 levels showed an increasing trend with progression from asymptomatic phase to AIDS phase. The mean levels for total IgG, IgG1 and IgG2 remained constant while that for IgG4 decreased with progression to AIDS phase. From the findings, it is concluded that polyisotypic hypergammaglobulinaemia and serum IgG subclass changes occur in HIV-1 seropositive individuals in Tanzania. The characteristic changes observed for IgD, IgG3 and IgG4 levels with evolution from asymptomatic to AIDS phases could be of value in monitoring disease progression.