Platelet activation initiates platelet-platelet aggregation, platelet-extracellular matrix interactions and thrombus formation. Platelet aggregation requires activation of the αIIbβ3 integrin, an event regulated by the integrin cytoplasmic tails. CIB1 binds to the cytoplasmic tail of the integrin αIIb subunit. CIB1 has been demonstrated to regulate αIIbβ3 activation in an acute setting. We studied the Cib1- /- mouse to determine if the chronic loss of CIB1 causes a defect in hemostasis. Plasma analysis exposed several significant differences in protein levels between the Cib1+/+ and Cib1-/- plasma. Most notable were the increases in von Willebrand factor and soluble P-selectin. To understand the effect of two pro-thrombotic proteins circulating at higher levels in the plasma, we examined bleeding time and arterial thrombus formation discovered no significant difference in thrombus formation or stability between the Cib1+/+ and the Cib1-/- mice. Previous overexpression and knockdown studies in murine megakaryocytes from our lab demonstrated that CIB1 inhibits platelet integrin αIIbβ3 activation. We analyzed Cib1-/- mice to determine the function of CIB1 in platelets in vivo. We found that although these mice had no overt platelet phenotype, mRNA level of CIB1 homolog CIB3 was increased in Cib1-/- megakaryocytes. In vitro binding experiments demonstrate that recombinant CIB1, -2 and -3 bound specifically to an αIIb cytoplasmic tail peptide. Subsequent protein modeling experiments indicated that CIBs 1-3 each have a highly conserved hydrophobic binding pocket. Therefore, the potential exists for compensation by these CIB family members for loss of CIB1, thereby preventing pathologic thrombus formation in Cib1-/- mice. Interestingly, the Cib1-/- mouse does have a phenotype that may provide a model for studying critical levels of plasma proteins required for normal hemostasis.