Metachromatic leukodystrophy and multiple sulfatase deficiency disorder are severe neurodegenerative diseases inherited as separate autosomal recessive traits. Arylsulfatase A (aryl-sulfate sulfohydrolase, EC 3.1.6.1) activity is deficient in both diseases but in multiple sulfatase deficiency disorder, activities of arylsulfatases B and C and other sulfatases are also reported to be reduced. Somatic hybrid cell clones produced by fusing cultured fibroblasts from patients with these diseases were isolated by a nonselective technique based on unit-gravity sedimentation. Arylsulfatase A activity was restored in these hybrids. The complemented enzyme resembled the normal arylsulfatase A in heat stability, pH optimum, Km, electrophoretic mobility, and immunologic reactivity. Because a structurally normal enzyme can be restored in a hybrid only though intergenic complementation, these results indicate that the mutations responsible for the deficiency of arylsulfatase A activity in metachromatic leukodystrophy and multiple sulfatase deficiency disorder are nonallelic and that at least two genetic loci control the expression of arylsulfatase A activity in the human genome. Furthermore, arylsulfatase C activity was also restored to normal in the hybrids, indicating that a common sulfatase inhibitor is not the cause of the multiple sulfatse deficiency.