Contacts between HIV-producing T cells and primary CD4+ T cells may induce the uptake of HIV by target cells that are endocytosed into trypsin-resistant compartments. We have now compared the mechanism of virus transmission from T cell-to-T cell versus infected dendritic cells (DCs)-to-T cell. In cocultures of HIV-1-infected DCs with primary CD4+ T cells, virus transmission to the target cells was resistant to trypsin treatment and could only be prevented by the anti-SUgp120 antibody IgGb12 but not by TAK-779, C34 or AZT. Importantly, upon stimulation of purified HIV-1-loaded CD4+ T cells with PHA/IL-2, cells became productively infected as measured by intracellular CAp24 staining and antigen determination in the cell supernatant. These results suggest that the viral endocytic transfer may represent a escape mechanism in the presence of drugs targeting HIV-1 entry or the host immune system.