A method was developed to directly detect glutathione S-transferase (GST) on the surface of a gel-plate after electrophoresis. First, a photograph of the background photoabsorption of the gelplate was taken by exposing photographic paper laid under the gel-plate to ultraviolet light (365 nm) without any chemical treatment of the gel surface. Next, the same plate was evenly coated with a solution consisting of 3 ml of 20 mM reduced glutathione (GSH) in water and 0.3 ml of 30 mM 1-chloro-2, 4-dinitrobenzene (CDNB) in ethanol. This plate was observed under ultraviolet light (365 nm), and photoabsorbing spots were directly photographed by the same method as before. Finally, the same plate was stained with Coomassie Brilliant Blue R-250 to give a protein picture. The mutual comparison of these three electrophoretic pictures made it possible to detect GSTs despite the presence of many other proteins on the gel-plate.