Our studies have focused on the role of monocyte/macrophages and accessory cells (reticuloendothelial system) in the pathogenesis of AIDS. An in vitro cell system of peripheral blood (PB)-derived monocyte/macrophages has been developed. It has been established that PB-derived monocyte/macrophages are as susceptible targets for HIV-1 as PB-derived T-cells. Moreover, this cell system can be successfully applied for virus isolation in situations where T-cell systems fail. Using the monocyte/macrophages as targets, HIV-1 has been isolated from cells of the mononuclear phagocyte lineage from various tissues. These isolates are being characterized with respect to biological behavior, nucleic acid properties of the viral genome, and viral protein expression in monocyte/macrophages vs. T-cells.