We have previously shown that erythroleukemia cells (K562) transfected with vascular adhesion molecule 1 (VCAM-1) are susceptible to human T-cell leukemia virus type 1 (HTLV-1)-induced syncytium formation. Since expression of VCAM-1 alone is not sufficient to render cells susceptible to HTLV-1 fusion, K562 cells appear to express a second molecule critical for HTLV-induced syncytium formation. By immunizing mice with K562 cells, we have isolated four monoclonal antibodies (MAbs), K5.M1, K5.M2, K5.M3, and K5.M4, that inhibit HTLV-induced syncytium formation between infected MT2 cells and susceptible K562/VCAM1 cells. These MAbs recognize distinct proteins on the surface of cells as determined by cell phenotyping, immunoprecipitation, and Western blot analysis. Since three of the proteins recognized by the MAbs appear to be GPI linked, we isolated lipid rafts and determined by immunoblot analysis that all four MAbs recognize proteins that sort entirely or in large part to lipid rafts. Dispersion of lipid rafts on the cells by cholesterol depletion with beta-cyclodextrin resulted in inhibition of syncytium formation, and this effect was not seen when the beta-cyclodextrin was preloaded with cholesterol before treating the cells. The results of these studies suggest that lipid rafts may play an important role in HTLV-1 syncytium formation.