The αIIb Y190F and D232H mutations increase the RUC-1 IC50 for fibrinogen. Fibrinogen binding to cells expressing normal human αIIbβ3, the αIIb Y190F mutant, or the αIIb D232H mutant receptor in the presence of the activating monoclonal antibody PT25-2 was determined. The data were normalized and the mean ± standard error of the mean (SEM) for each concentration of RUC-1 tested is depicted.