A patient with a cerebro-hepato-renal syndrome was investigated. The visceral manifestations were those of the Zellweger syndrome (ZS); however, the child exhibited muscular hypertonia and survived into the 2nd year of life. Ultramicroscopically, hepatocytes were lacking peroxisomes, but, contrary to findings in one patient with ZS [2], contained smooth endoplasmic reticulum. No catalase was found by histochemistry or spectroscopy. Mitochondria showed normal succinate and glutamate respiration, and normal coupling of respiration to the phosphorylation potential. The cytochrome (cyt) content was diminished to one-third with an abnormally inversed redox pattern of the respiratory chain in the controlled state, cyt b being 5%, cyt c 23% reduced. The oxygen affinity of cyt a3 was normal. These findings exclude a defect in the nonheme iron protein region of the respiratory chain as described in ZS [2], but point to a functional abnormality of cyt b in out patient.