The C-type lectin dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN) is found on the surface of dendritic cells. It can mediate adhesion between dendritic cells and T lymphocytes and facilitate antigen capture and presentation. Many pathogens can exploit DC-SIGN binding for nefarious purposes. For example, DC-SIGN can facilitate the dissemination of viruses, like HIV-1. Alternatively, some microbes (e.g., Mycobacterium tuberculosis) use their ability to interact with DC-SIGN to evade immune detection. The diverse roles attributed to DC-SIGN provide impetus to identify ligands that can be used to explore its different functions. Such compounds also could serve as therapeutic leads. Most of the DC-SIGN ligands studied previously are mannose- or fucose-derived monosaccharides or oligosaccharides with inhibitory constants in the range of 0.1 -10 mM. To identify monovialent ligands with more powerful DC-SIGN blocking properties, we devised a high-throughput fluorescence-based competition assay. This assay afforded potent non-carbohyd rate, small molecule inhibitors (IC50 values of 1.6-10 mu M). These compounds block not only DC-SIGN-carbohydrate interactions but also DC-SIGN-mediated cell adhesion. Thus, we anticipate that these non-carbohyd rate inhibitors can be used to illuminate the role of DC-SIGN in pathogenesis and immune function.