A small percentage of women at high risk for human immunodeficiency virus (HIV) exposure remain uninfected for long periods, protected by unknown mechanisms. We hypothesized that one mechanism could be inhibition of interactions between HIV and dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN) in the genital tract. In an analysis of 95 cervicovaginal lavage samples, we found that 12 (12.6%) strongly inhibited the binding of laboratory-adapted and primary HIV-1 isolates to B-THP-1/DC-SIGN cells in a dose-dependent manner, independently of the donor's risk of exposure. Three of 5 primary isolates were also blocked from binding to primary DCs. The inhibitor has a high molecular weight, is heat stable, and is resistant to trypsin. It is sensitive to pronase and periodate, indicating that it is likely a glycoprotein. Mannosidase digestion and concanavalin A adsorption indicate that the terminal residues of the carbohydrate are not mannose. Mechanistic experiments indicate that the inhibitor acts via binding to DC-SIGN. Further study of such inhibitors may help to elucidate the role played by DC-SIGN in HIV transmission.