Membrane glycoprotein (GP) IIb-IIIa is a component of a platelet adhesive protein receptor. A region of the heavy chain of GPIIb, defined by the monoclonal antibody PMI-1, is involved in adhesion receptor function. We have localized and chemically synthesized this region of GPIIb. A cDNA clone that directs the synthesis of a fusion protein reactive with the PMI-1 antibody was isolated from a phage lambda gt11 expression library constructed with mRNA from an erythroleukemia (HEL) cell line. The deduced amino acid sequence of this clone indicates that it spans the light-heavy chain junction of GPIIb and contains a portion of the carboxyl terminus of the heavy chain and the amino terminus of the light chain. The PMI-1 epitope was found to be contained within a 9-kDa staphylococcal V8 protease fragment of GPIIb, and such a fragment was predicted within the putative heavy-chain sequence. A computerized antigen prediction program identified a single sequence with a high probability of containing a continuous epitope. A synthetic 17-residue peptide containing this sequence binds PMI-1 and inhibits PMI-1 binding to GPIIb-IIIa. The peptide-antibody complex has an approximate Kd of 1.2 microM, which compares to a Kd of 0.95 microM for PMI-1 binding to GPIIb. The region containing the PMI-1 epitope shows no similarity to corresponding regions of two other adhesion receptors, indicating that this portion of GPIIb may function in activities unique to the platelet receptor.