We have characterized the IgG form of a previously isolated and engineered single-chain Fv (scFv), named RR2r3s4-1, that binds to human PSGL-1. This fully human IgG was determined to have a Kd of 1.8+/-0.7 nM by fluorescence quenching titration. It better inhibits P-selectin-PSGL-1 interactions than a commercially available murine monoclonal antibody KPL1 and better inhibits neutrophil rolling than KPL1. Thus, RR2r3s4-1 is the most effective antibody at inhibiting P-selectin-PSGL-1 interactions known. Specificity analysis reveals that RR2r3s4-1 does not cross react with murine PSGL-1 and thus requires more than tyrosine sulfate for binding to human PSGL-1. This evidence demonstrates the therapeutic potential of this antibody as a potent anti-inflammatory therapeutic.