Dendritic cells (DC) are the principle APC involved in primary immune responses; their major function is to obtain Ag in tissues, migrate to lymphoid organs, and activate T cells. DC are also the first immune cells to arrive at sites of inflammation on mucous membranes, the major site of sexual transmission of HIV. We have demonstrated previously that three populations of cells that can develop a dendritic morphology are present in peripheral blood. Two of these populations can express CD83, a marker of DC, and appear to be at different stages of maturation: 1) a precursor population and 2) a mature immunostimulatory DC. Precursor-derived DC express high levels of CD86 (B7-2) and HLA-DR but no CD80 (B7-1), whereas mature DC have high levels of expression of all three markers. Mature DC in peripheral blood bind HIV to their surface and induce infection when added to autologous CD4+ T cells in the absence of added stimuli, such as mitogens. These mature DC, when isolated directly from peripheral blood, appear to be conjugated to T cells, and these conjugates are infected easily and productively with HIV. These findings suggest a role for DC in early HIV infection in which they bind virus and interact with T cells locally or after migrating to a lymphoid organ, thus establishing a productive infection. Furthermore, they likely play a role in the propagation of HIV infection by activating T cells in the presence of HIV, which leads to viral replication and immune cell destruction.