It has been previously shown that skin biopsies isolated from various xeroderma pigmentosum (XP) patients present a permanent decline in catalase activity from the onset of the disease to the tumor formation. We report here that cultured XP cell strains are also markedly deficient in the catalase activity with about only 25% of the activity measured in normal human cells. No direct correlation between catalatic activity and excision repair ability has been found, since a XP variant line is as deficient as an XP-C strain. The exact cause of the catalase deficiency is still unknown but could be due to the synthesis of a modified enzyme or to an abnormal regulation leading to a limited enzyme synthesis. Furthermore, simian virus 40 transformation of normal and radiosensitive cells (XP, ataxia telangiectasia) provokes a decrease in catalase activity of about 80% compared to the control derivatives. Mathematical analysis performed on our data shows a clearcut distinction between XP and normal cells while some of the XP heterozygote cells exhibit an intermediate behavior. Although most of the XP syndrome could be explained by the impairment in the excision repair ability, the decrease in catalase activity leading to a probable increase in intracellular H2O2 concentration and/or to a higher sensitivity to any oxygen-activated species could represent an additive effect in inducing the carcinogenic process.