The human immunodeficiency virus type 1 (HIV-1) genome codes for trans-activator Tat, an 86-residue protein whose expression is critical for viral replication. Full-length Tat and Tat peptides from HIV-1 were chemically synthesized using optimized solid phase technique. Synthetic Tat2-86 was found not only to inhibit antigen-induced human peripheral blood lymphocyte (PBL) proliferation in vitro, as described by Viscidi et al. [1989, Science 246, 1606-1608], but also mitogen-induced PBL proliferation, with 50% inhibition obtained at 0.9 and 8 microM, respectively. To assess the mechanism by which Tat exert its inhibitory effect, we analysed its interaction and effect on CD4(+)-cells. Direct fluorescence and indirect immunofluorescence assays analysed by flow cytometry showed that fluorescein isothiocyanate-labeled and -unlabeled Tat interact (> 0.2 microM) with CD4-expressing lymphoid cells (CEM cell line). Experiments of chromium-51 release and Trypan blue exclusion on these tumor cells in vitro have demonstrated the capacity of Tat to modify cellular membrane permeability and cell viability, in a dose-dependent manner. The use of Tat peptides revealed that those containing the Tat basic region from 49 to 57 were able to bind to the cell membrane and to exhibit a cytotoxic activity on lymphocytes. Together, the data suggest that the potential cytotoxicity of Tat on lymphocytes could be directly implicated in virus-induced immune dysfunction observed in HIV-1 infected patients.