Peroxisomes are among the intracellular organelles of eukaryotic cells that contain specialized sets of enzymes with specific functions. Little is known of membranous components involved in assembly of the intracellular compartments. We isolated two peroxisome-deficient and mutually complementary, Chinese hamster ovary cell mutants, Z65 and Z24, which closely resembled fibroblasts from patients with autosomal recessive, peroxisome-defective disorders such as Zellweger syndrome. These patients show characteristic dysmorphism, severe hypotonia, psychomotor retardation, and peroxisomal dysfunctions and rarely survive early childhood. Here we report what seems to be the first direct cloning and characterization of a complementary DNA encoding a peroxisomal membrane protein of relative molecular mass 35,000 (Mr35K) that restores the biogenesis of peroxisomes and complements the defect of peroxisomal functions in the mutant Z65.