mAb 314.1 recognizes the same, or a very similar epitope on GPIIb, in the absence and presence of quinine. Saturating quantities of the mAbs listed on the abscissa were added to platelets and binding of Alexa Fluor 488 labeled mAb 314.1 in the presence (top) and absence (bottom) of quinine was then measured. Both quinine-dependent and quinine-independent binding of mAb 314.1 was blocked by mAbs 10E5 and 290.5, known to be specific for the N terminus of the GPIIb β propeller domain recognized by mAb 314.1. In contrast, binding of mAb 314.1 was relatively unaffected by mAbs 312.8 and AP3, which bind elsewhere on GPIIb and GPIIIa, respectively. Irrelevant mAb MOPC had no effect. Findings shown are representative of 3 independent studies that yielded similar results.