Thymus and thymocytes peroxidizability was studied in well defined conditions where lipid hydroperoxide dependent peroxidation occurs. The results show that: (i) lipid peroxidation is very low in thymocytes notwithstanding the higher content of arachidonic acid; (ii) the amounts of lipophilic chain-breaking antioxidants is higher in thymocytes; (iii) the thymus contains more total lipids and phospholipids. Thus the higher sensitivity to peroxidation of the thymus can be due to the replacement of thymus parenchyma by fatty tissue not correlated to an increase of lipophilic antioxidants.