HK blocks ¹²⁵I-MoAb SZ 2 (anti-GP Ib) binding to platelets. The binding of the anti-GP Ib-IX-V MoAb ¹²⁵I-SZ 2 (53 nmol/L) incubated with gel-filtered platelets was measured in the presence of increasing concentrations of HK. Unlabeled SZ 2 added at 0.88 μmol/L was assessed for its ability to reduced ¹²⁵I-SZ 2 binding compared with ¹²⁵I-SZ 2 alone, which was set equal to 100%. The results are the mean ± SEM of quadruplicate replicates of a single experiment. All points, compared with SZ 2 alone, were significant (P < .0005).