Some biochemical properties of the catalase inhibitor purified from maize scutella are described. The inhibitor is heat-labile and its activity is destroyed by trypsin, indicating that it is a protein. It does not appear to be a lectin nor does the inhibition involve proteolysis. The active inhibitor is a dimer with each subunit having a molecular weight of 5600 as determined by sodium dodecyl sulfate electrophoresis. A kinetic analysis performed in the presence of increasing levels of inhibitor gave unusual Lineweaver-Burk patterns. Possible explanations for these patterns are discussed. The inhibitor is active against all catalases tested from a wide variety of organisms.