We have developed a chemical modification of antibodies, lipidation, which enables their intracellular delivery into living cells. Intracellular localization of lipidated antibodies was demonstrated by confocal microscopy and by measuring cellular uptake of 125I-labeled lipidated antibodies. Functionally, a lipidated monoclonal antibody directed against the Tat protein from human immunodeficiency virus type 1 (HIV-1) inhibited viral replication of several HIV-1 isolates by approximately 85% as shown by increased viability of infected cells and decreased reverse transcriptase activity. The antibody in its native form had no such effect. These data show that lipidated antibodies can reach and functionally inhibit intracellular targets. Lipidation may help to facilitate the development of intracellular immunotherapy for AIDS.