Investigations mapped the region(s) on the light chain of high molecular weight kininogen (HK) that participates in cell binding. Sequential and overlapping peptides of domain 5 (D5H) were synthesized to determine its cell binding site(s). Three peptides from non-overlapping regions on D5H were found to inhibit biotin-HK binding to endothelial cells. Peptides GKE19 and HNL 21 weakly inhibited biotin-HK binding with IC50 of 792 and 215 microM, respectively. Peptide HKH20 inhibited biotin-HK binding with an IC50 of 0.2 microM. Two peptides, GGH18 and HVL24, which overlapped HKH20, also inhibited biotin-HK binding to endothelial cells with IC50 values of 108 and 0.8 microM, respectively. Biotinylated HKH20 directly bound to endothelial cells. HK and HKH20 bound at or near the same site on endothelial cells because HK inhibited biotin-HKH20 binding (IC50 = 0.2 microM). A polyclonal anti-HKH20 antibody also blocked biotin-HK binding. Peptides HKH20 and HVL24 and anti-HKH20 antibody also inhibited the procoagulant activity of plasma HK. These data indicated that the cell and artificial surface binding sites on D5H overlap. The orientation of HK on endothelial cells may be critical for the assembly and activation of contact system enzymes and the expression of kininogen's anti-thrombin activity.