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Normal human erythrocyte catalase, when isolated by the method of Mörikofer et al. [Eur. J. Biochem. 11 (1969) 49–57], is heterogenous in two respects: (a) there are alternative molecular forms; (b) besides the active tetramer, enzyme dissociation products (dimer, monomer) are present. By means of exclusion chromatography on Sephadex G‐150 it was shown that catalase preparations contain dimer and monomer in small amounts (∼ 4%). Upon storage, the yield in subunits can be considerably enhanced (up to 15%). Residual catalase activity in the blood of an individual homozygous for Swiss Type Acatalasemia (1–2% of normal level) consists of a catalase variant, which is unstable, but of approximately normal specific activity. In this mutant, the tendency of dissociation into subunits is greatly enhanced. Upon purification, little tetramer but appreciable amounts of the inactive dimer are obtained. The antigenic properties of these molecular species were investigated using anti‐catalase and anti‐catalase‐subunit immunoglobulins G. There is complete antigenic identity between the normal and the variant catalase; the same is true for the normal and variant dimer. Different antigenic properties of dimer and monomer, however, offer evidence for the existence of hidden determinants in the subunits.

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