Monoclonal antibodies were raised against platelet integrins purified by affinity chromatography. Two monoclonal antibodies (5C5 and 3H8) reacted with the purified alpha 2 subunit of VLA-2 in Western blotting. The monoclonal antibody 3H8 also reacted with fibrinogen in Western blots and in ELISA tests. CNBr fragmentation of the alpha chain of fibrinogen generated two peptides which were still recognized by this monoclonal antibody in Western blotting. N-terminus sequencing of these two fragments showed that they were non-overlapping fragments of the fibrinogen alpha-chain with as only common epitope an RGD(F)/RGD(S) sequence. Dot blots and ELISA tests showed that the antibody 3H8 also recognized, however with lower affinity, fibronectin and collagen IV, which are RGDS containing extracellular matrix proteins. The assumption that Mab 3H8 recognizes an RGD sequence, was further supported by the findings that the binding of Mab 3H8 to fibrinogen was partially inhibited by RGDF containing peptides and that the antibody was able to inhibit platelet aggregation.