Circularly polarized luminiscence (CPL) from dilute solutions of Tb(III) bound to the Ca-binding protein calmodulin is reported. The dissymmetry ratio, gem, at 543.5 nm can be monitored as a function of equivalents of metal ion concentration. Competitive and consecutive addition of Ca(II) versus Tb(III) yield results which are consistent with previous results that suggest that Tb(III) and Ca(II) have a preferred affinity for different metal-ion binding sites in this protein.