Human T-cell leukemia virus type-I (HTLV-I) and human immunodeficiency virus type-1 (HIV-1) have a similar tropism for target cell types, especially for CD4+ T cells. Although CD4 is an HIV-1 receptor, the cellular receptor for HTLV-I is not yet known. HTLV-I does not effectively infect cells as a cell free virus, but require a cell-to-cell contact between HTLV-I infected cells and target cells. Initially, we used a syncytium formation system to detect HTLV-I infection. This system provided us an evidence that the receptors for HTLV-I and HIV-1 are totally independently present on the cell surface of human T cells. Furthermore, HTLV-I receptor seemed not to be involved in the CD4 molecule. We tried to make monoclonal antibodies for the HTLV-I receptor by using this system, but an antibody blocking HTLV-I-induced syncytium formation was not obtained yet. Besides such cell-to-cell system, we established an experimental system to detect HTLV-I adsorption, since the cell-to-cell would be more affected by adhesion molecules on the cell surface unrelated to virus receptors. HTLV-I particles were concentrated and incubated with the target cells' and viral adsorption to the cell surface was quatnitated by FACS analysis. HTLV-I could bind a wide range of human cell lines with a few exceptions. Human anti-HTLV-I sera inhibited the HTLV-I adsorption, suggesting that these sera contain antibodies neutralizing HTLV-I binding. Efforts to obtain monoclonal antibodies to the HTLV-I receptor are currently being made using these systems.