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Fibrinogen Paris I contains a mutant 7 chain that is longer than the normal chain, resulting in altered fibrin polymerization and cross-linking. Because these functions involve the carboxy-terminal region of the  γ chain, we decided to determine whether fibrinogen Paris I or the isolated Paris I γchain supports normal ADP-induced platelet aggregation, a function that requires the ultimate 12 residues of the normal γchain (400 through 411). Aggregation of ADP-stimulated normal platelets was defective with fibrinogen Paris I and markedly depressed with the γParis I chain. These findings prompted us to characterize the carboxy-terminal structure of the region of the γParis I chain responsible for this activity. The carboxy-terminal cyanogen bromide (CNBr) peptide of the normal γchain (385 through 411) or that from γParis I was isolated by differential adsorption to triethylene-tetramine resin or by reverse-phase high-performance liquid chromatography (HPLC). The CNBr peptide from the Paris I γchain was identical to that of the normal γ chain in its retention time on HPLC, its amino acid composition, and its sequence. Thus, the primary structure of the γ Paris I chain from residue 384 through 411 is normal, indicating that a peptide insertion has occurred upstream from residue 384, resulting in an impairment of those physiologic functions attributable to the carboxy-terminal end of the γ chain from position 384 (ie, cross-linking, ADP-induced platelet aggregation, and at least a portion of the γ chain polymerization site). These observations demonstrate that the γ chain platelet recognition site in the fibrinogen molecule is necessary but not alone sufficient to support normal ADP-induced platelet aggregation. There appears to be an additional requirement for normal conformation of the 7 chain or availability of its terminal sequence during the interaction of fibrinogen with platelets.