?-galactosidase is a homo-tetrameric enzyme, with each subunit having a molecular weight of 116 kDa. Eukaryotic genes are often expressed as fusion protein by the ?-galactosidase (lacZ) gene, resulting in the expression of a fusion hybrid with ?-galactosidase. Anti-?-galactosidase antibody provides a simple method to isolate fusion proteins directly from crude bacterial lysates, using immunoaffinity chromatography or immunoprecipitation. Anti-?-galactosidase can also be used for the immunocytochemical detection of ?-galactosidase in cells and tissues that express transfected bacterial lacZ gene or ?-galactosidase fusion protein.