Monoclonals 314.1 and 314.3 reacted with isolated GPIIb/IIIa in the presence, but not in the absence of quinine. Platelet GPIIb/IIIa was captured from a platelet lysate by monoclonal AP2 (anti-GPIIb/IIIa complex) immobilized in microtiter wells. Biotinylated mAbs 314.1 and 314.3 were incubated with this target in the presence (■) and absence (□) of 0.4 mM quinine. Bound mAb was detected with alkaline-phosphatase-labeled streptavidin. Brackets show the mean plus or minus 1 standard deviation of triplicate determinations.