In metachromatic leucodystrophy abnormal lipids are stored within the lysosomes of the glial cells and phagocytes. The lipid inclusions which are the less structured are the richest in acid phosphatases. Those of which the ultrastructure is prismatic show a very low enzymatic activity and are more similar to residual bodies than to active lysosomes. It appears that the lack in lysosomal arylsulfatase leads to an accumulation of the sulfatide moiety of the stored lipids while the other lipids originating from the myelin breakdown, would be normally catabolized. This leads to a progressive transformation of the ultrastructure of the lysosome contents.