Calcium- and integrin-binding protein 1 (CIB1) regulates platelet aggregation in hemostasis through a specific interaction with the aIIb cytoplasmic domain of platelet integrin aIIbß3. In this work we report the structural characteristics of CIB1 in solution, and the mechanistic details of its interaction with a synthetic peptide derived from the aIIb cytoplasmic domain. NMR spectroscopy experiments using perdeuterated CIB1, together with heteronuclear NOE experiments have revealed a well-folded a-helical structure for both the ligand free and aIIb-bound forms of the protein. Residual dipolar coupling experiments have shown that the N- and C-domains of CIB1 are positioned side-by-side, and chemical shift perturbation mapping has identified the aIIb-binding site as a hydrophobic channel spanning the entire C-domain and part of the N-domain. Data obtained with a truncated version of CIB1 suggest that the extreme C-terminal end of the protein weakly interacts with this channel in the absence of a biological target, but it is displaced by the aIIb cytoplasmic domain, suggesting a novel mechanism to increase binding specificity.