Catalase from a crude preparation of Penicillium chrysogenum was isolated in a single chromatographic step by immobilized metal ion affinity chromatography (IMAC) on Cu(II)-Chelating Sepharose Fast Flow. A chromatographically and electrophoretically homogeneous enzyme was obtained in 89% yield. IMAC was found to be superior to ion-exchange, hydrophobic interaction, size-exclusion and concanavalin A affinity chromatography. Analytical and preparative chromatography gave essentially the same chromatograms. Isoelectric point, molecular weight (by ultracentrifugation), amino acid composition, carbohydrate content and subunit organization were determined. The apparent Michaelis-Menten constant, K(M), and the azide competitor constant, K(i), were calculated and found to be 59-mu-M and 6.1-mu-M, respectively.