Two peptides corresponding to the sequence of platelet glycoprotein IIIa between serine 27 and arginine 37 were synthesized and used to produce monoclonal antibodies. These two synthetic peptides were identical except for a single substitution at position 33, where a Pro/Leu polymorphism was shown to occur in human platelets and was predicted to be responsible for the P1A1-P1A2 alloantigen system (Newman et al., J. Clin Invest 1989:83:1778-81). Two monoclonal antibodies named 3C1 for the anti-"P1A1 peptide" and AD3 for the anti-"P1A2 peptide" were characterized. These monoclonal antibodies interacted with the two allelic forms of the reduced glycoprotein IIIa. They were used to type P1A1 and P1A2 homozygote as well as heterozygote platelets. Thus these two immunoprobes confirm that the Pro-Leu substitution is associated with the P1A1-P1A2 alloantigenic system. Although they interact only with reduced glycoprotein IIIa, these antibodies can be used to design simple tests for the typing of the P1A status of patients.