A recently developed sensitive assay to examine the early stages of HIV-1 env-mediated cell fusion is based on the redistribution of fluorescent dyes between membranes and cytoplasm of adjacent cells, monitored by fluorescence video microscopy. This assay demonstrated that membrane fusion can occur under conditions where no syncytia are formed. Fusion started earlier than syncytia formation and was not very sensitive to HIV-1 env+/CD4+ cell ratios. In the current study, this assay was used to determine the role of LFA-1 in HIV-1 env-mediated membrane fusion and syncytia formation. CD4- LFA-1- Epstein-Barr virus transformed lines from two leukocyte adhesion deficiency patients were infected with recombinant vaccinia expressing gp120/41 (HIV-IIIB), and cocultured with CD4+ subclones of the human T cell line CEM, which were generated by chemical mutagenesis and express either normal (LFA-1+), or low levels of LFA-1 (LFA-1lo). It was found that the LFA-1lo T-cell clone formed much smaller and fewer syncytia compared to the LFA-1+ subclones, but both clones fused equally well with the gp120/41 expressing LFA-1- B cells as monitored by redistribution of fluorescent dyes. Furthermore, monoclonal antibodies against the LFA-1 molecules reduced the number of syncytia formed but had no effect on membrane fusion. These findings demonstrate that the adhesion molecule LFA-1 does not play a crucial role in the early events of HIV-1 env-mediated cell membrane fusion, but may contribute to the later events leading to giant cell formation.