The study on the interaction of BPLA 2 with Ca 2 + by fluorescence spectroscopy indicated that Ca 2 + was very important to BPLA 2 . In the presence of Ca 2 + , the environment around the Trp residue in BPLA 2 became more hydrophobic upon the binding of the substrate to the enzyme, resulting in 9 nm blue shift of the Trp fluorescence emission spectrum. But such a phenomenon was not observed in the absence of Ca 2 + . The experiment indicated that there was a hydrophobic region in BPLA 2 molecule. Ca 2 + might increase the hydrophobility of this region. Further, it was also found that the binding of Ca 2 + to the enzyme was related to the single His residue of the enzyme.