Porcine brain glutamate decarboxylase interacts with derivatized phopholipid fluoreceine-dtpahtdtoyl-phosphatldyl ethanolamine (FL-DPPE). Upon addition of the enzyme to FL-DPPE, an increase in fluordcence intensity is discernible, together with an increase in emission anisotropy. Judging from anisotropy values, FL-DPPE is rapidly trapped by the protein matrix The binding of FL-DPPE to glutamate decacboxylese on of the dimeric enzyme into polymeric species. It appears that 2.2 moi of FL-DPPE ate bound per protein molecule, as determined by a study of fluorescence quenching. A quenching bimolecular rate constant of 1.1 × 10^9M^(-1)·S% was obtained from a Stem-Volmer plot. These results suggest that fatty add tails sense as anchoring sites for the protein dimers, whereas the fluorescent probe covalently bound to the hydrophilic head of the phospholipid, remains accessible to the collisional quencher.