A gamma-aminobutyric acid (GABA) binding protein (GBP) was isolated from a bacterial mutant which has high-affinity GABA binding characteristics comparable with the GABAA brain receptor in mammals. The GBP was partially purified and characterized and was shown to be a periplasmic protein of approximately 42,000 molecular weight. To determine the molecular weight, a bacterial GABA binding assay was used with SDS-PAGE. This procedure did not require large amounts or complete purification of protein and may be useful as a simple method in estimating the molecular weight of other bacterial binding proteins.