To identify platelet surface structures involved in adhesion to collagen, the effect of 16 murine antiplatelet membrane hybridoma antibodies were tested in a defined, in vitro assay. Four of these antibodies inhibited platelet-collagen adhesion and reacted with a polypeptide with Mr approximately 125,000, as determined by immunoblots after gel electrophoresis under reducing conditions. Through detailed studies with one of these antibodies, the monoclonal antibody PMI-1, the relevant antigen was identified as platelet glycoprotein IIb alpha, based upon (a) co-migration with this glycoprotein in two-dimensional gel electrophoresis and (b) co-purification by immunoaffinity chromatography with a protein with apparent Mr identical to that of glycoprotein III, under conditions in which glycoproteins IIb and III form a complex. Univalent antibody fragments prepared from monoclonal antibody PMI-1 inhibited greater than 80% of platelet-collagen adhesion, and inhibition was completely blocked by the immunopurified antigen. These results indicate that glycoprotein IIb participates in some aspect of platelet-collagen adhesion. In contrast, the purified antigen only partially neutralized a polyclonal antiserum that blocked platelet-collagen adhesion, to a maximum of approximately 25%, at saturating antigen concentrations. Thus, by these immunological criteria, glycoprotein IIb is not the only molecule involved in this process.