Human CD4+ T lymphoblastoid Jurkat cells were stably transfected with two different plasmid vectors containing the cDNA of human immunodeficiency virus-type 1 (HIV-1) tat gene under the control of either the promoter of simian virus 40 (pRPneo/tat) or the long terminal repeat region of SL3 murine leukaemia virus (pRPneo/SL3/tat). Both pRPneo/tat and pRPneo/SL3/tat Jurkat cell lines showed a constant and high production of bioactive Tat in transient co-transfection assays with an HIV-1 long terminal repeat (LTR)-chloramphenicol acetyltransferase (CAT) reporter plasmid. Tat-positive and mock-transfected Jurkat cells were cultured with various cytotoxic agents, which have been associated to the progressive loss of CD4 T-lymphocytes characteristic of HIV-1 disease. In the presence of recombinant tumour necrosis factor-alpha (TNF-alpha), anti-fas antibody, Leu3a anti-CD4 antibody, the percentage of apoptosis, evaluated in a 24-72 h short-term assay, was lower (P < 0.05) in tat-positive Jurkat cells than in mock-transfected controls. The low susceptibility to the cytotoxic activity of TNF-alpha and anti-fas antibody of tat-transfected cells was confirmed by counting viable cells up to 15 d of culture. Also, recombinant Tat protein was able to prevent the increase of apoptosis induced in mock-transfected Jurkat by TNF-alpha. Of note, tat-expressing cells showed a better survival with respect to mock-transfected control cells even when acutely infected with high doses (500,000 cpm of reverse transcriptase) of HIV-1 (strain IIIB) or treated with heat-inactivated HIV-1. These data demonstrate that the expression of the regulatory HIV-1 Tat protein is able to rescue Jurkat lymphoblastoid cells from apoptosis induced by a variety of cytotoxic agents. Since Tat protein expression is restricted to the initial phases of an active HIV-1 replication, the anti-apoptotic effect of Tat could have the physiological significance of selectively protecting HIV-1 producing cells from death, at least for the time necessary to allow virus production and spreading.