Catalases from 16 different organisms including representatives from all three phylogenetic clades were purified and characterized to provide a comparative picture of their respective properties. Collectively the enzymes presented a diverse range of activities and properties. Specific activities ranged from 20,700 to 273,800 units per milligram of protein and maximal turnover rates ranged from 54,000 to 833,000 per second. The effective concentrations of common catalase inhibitors, cyanide, azide, hydroxylamine, aminotriazole, and mercaptoethanol, varied over a 100- to 1000-fold concentration range, and a broad range of sensitivities to heat inactivation was observed. Michaelis-Menten kinetics were approximately followed only at the low substrate concentrations. At high H(2)O(2) concentrations, inactivation of small-subunit enzymes resulted in lower velocities than what were predicted, whereas large-subunit enzymes had velocities higher than predicted. Kinetic constants such as K(m) and V(max) for catalases must be labeled as "apparent."