The human immunodeficiency virus-1 (HIV-1) Tat protein has previously been shown to transactivate the HIV-1-LTR when added exogenously to HeLa, H9 lymphocytic and U937 promonocytic cells growing in culture. Here we show that Tat enters these cells by adsorptive endocytosis. Tat appears to bind non-specifically to the cell surface, with greater than 10(7) sites per cell. A specific receptor was not detected by protein crosslinking experiments, and uptake was not affected by treating cells with trypsin, heparinase or neuraminidase. Uptake and transactivation could be inhibited by incubation with heparin, dextran sulfate, an anti-Tat monoclonal antibody, or by incubation at 4 degrees C. In contrast, transactivation by Tat was markedly stimulated by the addition of basic peptides, such as Tat 38-58 or protamine. Fluorescence experiments with rhodamine-conjugated Tat show punctate staining on the cell surface and then localization to the cytoplasm and nucleus. The lack of a specific receptor makes it unclear whether Tat uptake is biologically important in HIV infection, however, the efficiency of uptake raises the possibility that Tat may be useful for delivery of protein molecules into cells.