We have analyzed the reactivity of a new mouse monoclonal antibody (mAb), 11C3, which identifies a cell marker detected on the surface of chicken thrombocytes. Tissue distribution studies have shown that only cells of the thrombocytic lineage in blood, spleen, and bone marrow are stained by 11C3. However, it does not react with other species such as quail, mouse, and man. The 11C3 mAb immunoprecipitates an heterodimeric molecule made of two bands with an apparent molecular weight of 112 and 90 kDa under nonreducing conditions and 112 and 26 kDa following reduction. This pattern of migration is similar to the one observed for members of the integrin family of cell adhesion molecules. We have used the previously described mAb AP-2, which is specific for the human platelet integrin GPIIb-IIIa and cross-reacts with chicken thrombocytes. We have shown that it immunoprecipitates two bands with an identical electrophoretic mobility. Cross-inhibition and immunodepletion studies reveal that the two antibodies recognize two different isoforms or two conformational variants of the same molecule. Moreover, our data demonstrate that in contrast with AP-2, 11C3 is a potent inducer of thrombocyte activation measured by cell aggregation, chemiluminescence, or release of [³H]serotonin. It also inhibits the adhesion of thrombin-activated thrombocytes to fibrinogen and, to a lesser degree, to fibronectin, in a dose-dependent manner. Altogether, these results indicate that this antibody identifies the avian homolog of the mammalian platelet integrin and fibrinogen receptor GPIIb-IIIa.