Peroxisomes (microbodies) have been localized in the cytoplasm of various cells, mainly the hepatocytes and the renal tubular cells. Various enzymes were found in the granular matrix and sometimes in the nucleus ; they were involved in the production and the degradation of hydrogen peroxide, in regulation of the energetic processes of the cell, and in the oxidation of fatty acids. Several xenobiotics and drugs such as hypolipidemic agents clofibrate-like, have induced in rodents a proliferation of peroxisomes and frequently, the appearance of malignant tumors, in men, the precise role of peroxisomes in the physiology of the cell, their link with the hypolipidemic action of some drugs and the development of cancer remain to be established.