We have studied the urinary excretion of oxalate and glycollate in patients with the Zellweger syndrome and hyperoxaluria type I and have measured the activity of alanine glyoxylate aminotransferase (a peroxisomal enzyme in man) in the patients. In agreement with earlier reports we found that alanine glyoxylate aminotransferase was strongly deficient in liver from a hyperoxaluria type I patient, thus explaining the increased urinary excretion of oxalate and glycollate in these patients. In livers from Zellweger patients, however, in which morphologically distinguishable peroxisomes are absent, the enzyme was not deficient, which is in accordance with our finding that the urinary excretion of oxalate and glycollate was normal in these patients.