The transmission of HIV requires the interaction of the cell-surface CD4 receptor and the viral envelope glycoprotein. Experiments were performed to determine the role of other cell-surface molecules in the development of HIV-induced syncytia. Although CEM and MT-2 cells had similar cell-surface CD4 receptor densities, <1% of CEM cells and >95% of MT-2 cells formed syncytia with H9 cells chronically infected with HIV-1 (H9-IIIB). When compared with CEM cells, MT-2 cells exhibited a 10-fold and threefold greater capacity to form homotypic and heterotypic conjugates with H9 cells, respectively. Increasing the conjugate formation capacity of CEM cells with the lectin wheat germ agglutinin led to a >30-fold increase in the formation of syncytia with H9/IIIB cells. The formation of syncytia between MT-2 and H9/IIIB cells was magnesium-, energy-, temperature-, and actin-cytoskeleton-dependent, and could be inhibited (65%) by an anti-LFA-1 monoclonal antibody. The combination of anti-leukocyte function-associated antigen-1 (LFA-1) and anti-CD2 monoclonal antibodies resulted in a synergistic inhibition (89%) of syncytium formation. These results indicate that integrins and other cell-surface adhesion molecules regulate HIV-induced syncytium formation