Ca2+-binding affinity of sites III and IV in C-terminal domain of calmodulin in the presence and absence of BPA using intrinsic Tyr fluorescence is reported. The titration curves were lower shifted for Ca2+ affinity in the presence of BPA, and Gibbs energy of calcium binding to both sites decreased. These results suggest that BPA weakens the Ca2+ affinity of domains III and IV of calmodulin.