T lymphocyte subsets were determined on blood samples from 16 HIV-seropositive patients with manifest AIDS (CDC IV), 24 HIV-seropositive patients with lymphadenopathy syndrome (LAS, CDC III), 16 HIV-seropositive clinical healthy persons (CDC II) and 11 HIV-seronegative homosexuals as control group. Absolute numbers of T-cells (CD6+), T-helper/inducer-cells (CD4+) and T-suppressor/cytotoxic-cells (CD8+), obtained by immunofluorescence staining were compared with the absolute amount of subset specific surface molecules, obtained by a T-cell-ELISA. With both, indirect immunofluorescence technique and ELISA technique a highly significant decrease of the absolute numbers of CD4+ cells and the absolute amount of CD4 surface molecules, respectively, was found in asymptomatic HIV-infection, LAS and in manifest AIDS. In all HIV-seropositive groups the relative decrease of CD4 surface molecules was significantly greater than the decline of CD4+ cells. This phenomenon however was not seen in HIV-seronegative homosexuals. The absolute number of CD6+ cells and the amount of CD6 surface molecules were found significantly lowered in AIDS compared to HIV-seronegative homosexuals. No significant changes were found for CD8+ cell numbers and CD8 surface molecule in the progression of the HIV-infection.