We present in this paper a patient with a clinically intermediate form of mucolipidosis (ML). Lysosomal hydrolase activity in fibroblasts was normal and levels of these enzymes in culture media were not elevated. There was a striking elevation of several hydrolases in serum and a deficiency (15% of normal) of N-acetyl-glucosamine phosphotransferase in fibroblasts. Atypical electron microscopic findings were also observed. There was no evidence of increased synthesis, slower turnover, unbalanced distribution or further changes in lysosomal enzymes. Phosphotransferase deficiency against endogenous beta-glucosaminidase and the fact that the electrophoretic mobility of lysosomal enzymes was identical to that of MLII suggest that these enzymes are not phosphorylated. Hypotheses that could explain this atypical pathology are discussed.