The ultrastructure of dendritic cells (DC) isolated from HIV-negative blood donors revealed three morphologically distinct cell types: type I had an irregularly shaped nucleus with a high content of heterochromatin and numerous short pseudopodia; type II possessed a smoother boundary, a "blast-like" nucleus and a few veil-like protrusions; and type III resembled veil-like cells (DC) in the lymph. HIV induced a productive virus infection in type II DC with budding of virus solely from the cell surface. HIV was observed in cytoplasmic vacuoles of type III DC, but no budding of virus was observed from these cells. HIV was not observed in type I DC. Isolated monocytes were cultured for 8-15 days in order for these cells to differentiate into macrophages. Cultured monocytes and macrophages became highly vacuolized when infected with HIV strains and productive virus infection was localized in intracytoplasmic vacuoles. Budding from the cell surface was rarely observed. When monocytes were co-cultured with CD4(+)-infected lymphocytes, specialized electron-dense contact zones were observed in monocyte-like cells. Virus particles were found outside some of these contact zones, indicating that the zones may play a role in the penetration and transfer of virus from cell to cell.