Antisense oligodeoxynucleotide (ODN), which are directed against the splice acceptor site of exon II of the regulatory gene tat of the human immunodeficiency virus type 1 (HIV-1), have been described. These 20-mer ODN’s displayed moderate anti-HIV activity in vitro. Using the same antisense ODN (termed ODN-2), which was additionally modified and protected both at the 3’- and the 5’-terminus by two phosphorothioate internucleotide linkages, a strong anti-HIV activity (EC50: 2.7 µg/ml) could be measured in the HIV-1/CEM- and HIV-l/HeLa-T4+ cell system. The analogous ODNs which were protected only at one end were either inactive (up to 10 µg/ml) or displayed a low antiviral activity. Time kinetic studies revealed that the antisense ODN-2 reduced the release of HIV-1 already after an incubation time of 1 h. By applying SI nuclease protection procedures, it could be established that the antisense ODN-2 inhibited splicing of high molecular weight transcript to the 2-kb tat mRNA in HIV-1-infected CEM cells. Transfection experiments with pU3R-III chloramphenicol acetyltransferase expression vector in HeLa-T4+ cells revealed that the antisense ODN-2 blocked the Tat protein-mediated transactivation process. In co-transfection experiments using pSV2tat72 or scrape loading studies with purified Tat, the transactivation was restored. These data indicate that the selected antisense ODN-2 displays its anti-HIV effect by blocking the splicing process leading to the functional 2-kb tat <