The human hereditary disease Acatalasia (AC) is characterized by low or no catalase activity in all body tissues. We have studied the molecular basis of AC. In order to assess their antioxidant defense status we measured the enzyme activities, protein levels and m-RNA concentrations of catalase, superoxide dismutase and glutathione peroxidase in fibroblasts from a Japanese (AC65) and a Swiss (AC64) patient and several normal individuals. Our results point to genetic heterogeneity. While strain AC64 contained normal levels of catalase mRNA and -protein, strain AC65 was completely devoid of both. A structural mutation in the catalase gene is probably responsible for the inactivation of the enzyme in AC64. Since AC65 contains at least a major portion of the catalase gene it may represent a regulatory mutation in which the gene is not transcribed.