Based on the heterogeneity observed in two red cell enzymes, i.e. glucose 6-phosphate dehydrogenase (E.C. 1.1.1.49) and catalase (E.C.1.11.1.6), the role of different types of enzyme variants in enzyme deficiency conditions is discussed. For theoretical and practical reasons variants of unusually low specific activity and of low stability have to be distinguished. Whereas in the former type the activity level in blood more or less reflects the situation in other tissues, this is not the case for unstable mutants, e.g. the enzyme variant found in Swiss-type acatalasemia. In heterozygous carriers the situation can be complicated by the fact that variants of oligomer enzymes (e.g. catalase) are present as molecular hybrids exerting almost normal stability and activity.