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Calcium binding to bone γ-carboxyglutamic acid protein (BGB) from calf has been studied using 43Ca NMR. The temperature dependence of the 43Ca NMR signal has been used to calculate the calcium ion exchange rate, koff. The dependence of the 43Ca NMR band shape on the [Ca2+]/[BGP] ratio fits well to a chemical equilibrium model having a single Ca2+ -binding site with an association constant in the range of 5 × 103–1 × 105 M−1. The pH dependence of the 43Ca NMR line-width shows a single apparent pKa value of 5.1.