More than 18 compounds have been tested for their ability to stimulate defective UDP-glucuronyltransferase activity of Gunn rat liver homogenates towards 2-aminophenol, up to levels of transferase activity in similarly treated Wistar rat liver homogenates. The minimum structural requirements of an effective compound are a combination of the presence of an electron-attracting atom or group and a sparing solubility in water. The activation of defective UDP-glucuronyltransferase towards 2-aminophenol by pentan-3-one is reversible. The possible mechanism of action of alkyl ketone activators is discussed.