Biochemical and ultrastructural investigations were made in 2 children suffering from mucolipidosis type III. Among the lysosomal hydrolases the activity of beta-galactosidase and alfa-fucosidase diminished in the homogenate of the peripheral leukocytes in case I. The activity of serum and leukocyte arylsulfatase was normal. By electron microscopy typical storage organellums for mucolipidosis were detected in different biopsy materials--liver, skin, conjunctival ones--and in the cytoplasm of the peripheral lymphocytes and leukocytes. Definitive diagnosis was given by the specific electron microscopic investigations detecting the typical storage patterns for mucolipidosis.