3,3'-Diaminobenzidine strongly inhibits bovine liver catalase in two distinct ways. One of these was competitive with respect to H2O2, approached a limit of 100% inhibition and was rapidly reversed by dilution or by dialysis. The other was dependent upon H2O2, approached a limit of 60% and was not reversed by dilution or dialysis. Exposure to diaminobenzidine followed by dialysis did not modify the electrophoretic mobility of the enzyme, its Km for H2O2 or its optical spectrum, although Vmax. was halved. Diaminobenzidine protects catalase against the irreversible inactivation imposed by 3-amino-1,2,4-triazole plus H2O2. CN-protected catalase against the apparently irreversible inhibition by diaminobenzidine, and dialysis against CN- reversed this inhibition. Ethanol was similarly protective, and ethanol plus H2O2 reversed the inhibition by diaminobenzidine. Several other aromatic diamines inhibited in a fashion similar to that of diaminobenzidine, but were less effective. A scheme of reactions that accounts for these observations is proposed.