A simple and rapid method for determination of catalase activity in small tissue samples is described. Using a new approach, we have exploited the peroxidatic function of catalase for the determination of enzyme activity. The method was based on the reaction of the enzyme with methanol in the presence of an optimal concentration of hydrogen peroxide. The formaldehyde produced was measured spectrophotometrically with 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole (Purpald) as a chromogen. With this method, a detection limit of 12.5 ng of purified catalase from bovine liver was possible, and it was successfully applied to microgram amounts of mouse liver and pancreatic islet homogenates. The catalase activity in liver was about 50 times higher than that in pancreatic islets.