We have shown that a monoclonal antibody to the cell surface adhesion molecule LFA-1 (CD18/CD11a) enhances plasma neutralization of a laboratory isolate (HIVMN) and a primary isolate (HIV28R) of human immunodeficiency virus type 1. Human phytohemagglutinin blasts were infected with HIVMN or HIV28R in the presence of plasma pooled from HIV-positive individuals (AIDS plasma) or immunoglobulin G from AIDS plasma alone or combined with a monoclonal antibody (MAb) to LFA-1. While AIDS plasma alone at a dilution of 1:1,250 neutralized HIVMN and HIV28R infection by 15 and 0%, respectively, in the presence of a saturating concentration of the MAb to LFA-1 the plasma neutralized both viruses by more than 80% at this dilution. Immunoglobulin G purified from AIDS plasma, when used in combination with the MAb to LFA-1, showed the same synergistic effect in HIV neutralization as seen with the AIDS plasma and anti-LFA-1. The MAb against LFA-1 partially neutralized both viral isolates (45 to 55%) on its own. These results demonstrate significant synergy between the plasma and antibody against LFA-1 in the neutralization of HIV. The observations therefore suggest an important role for adhesion molecules in HIV infectivity and transmission. The results have implications for the recently observed host effect on HIV susceptibility to antibody neutralization.