Two clinical forms of lysosomal storage of neutral lipids with deficiency of acid lipase are known: the severe infantile form is called Wolman disease, whereas the more benign adult form is called "polycorie cholesterolique de l'adulte" or cholesteryl ester storage disease (CESD). We have developed several new tools for the study of hereditary enzymopathies and we report here their use in the study of genetic defects in lysosomal acid lipase. Lymphoid cell lines established by Epstein-Barr Virus transformation represent a new experimental model system in culture. The validity of such cell lines is demonstrated from the triple point of view of enzymology, metabolism and morphology (microscopic studies). Those lines are characterized by the deficiency in acid lipase whereas other lipases and carboxylesterases are not deficient. The study of lipid composition demonstrated the accumulation of neutral lipids (cholesterylesters and triglycerides). The use of fluorescent lipids for enzymatic studies and for diagnosing acid lipase deficiencies is reported and the reliability of these new substrates is demonstrated by comparison with currently used natural or synthetic substrates. For the enzymatic assays, these fluorescent substrates showed similar advantages as the radioactively labelled substrates, but not their disadvantages: thus, they can be easily used in the clinical laboratories. A new area in the field of research on lysosomal genetic diseases, is the experimental study of the metabolism in the intact living cell. The lipoproteins route is the best way to the lysosomal compartment and fluorescent lipids incorporated into lipoproteins allow to examine the lysosomal metabolism of these lipids simultaneously, by microscopic and biochemical techniques.