Arylsulfatase A (cerebroside sulfate sulfatase E.C.3.1.6.1.), a lysosomal hydrolase participating in the catabolism of sulfatide (cerebroside sulfate), is assayed in vitro by the synthetic chromogenic sulfate ester of phenol or by the natural sulfatide substrate [l]. Deficiency of arylsulfatase A (ASA) activity in humans leads to metachromatic leukodystrophy (MLD), a neuro-degenerative metabolic disease characterized by the accumulation of sulfatide in various tissues mostly in the nervous system. The disease is an autosomal recessive trait, and intermediate activity of ASA may be found in heterozygotes [l]. Healthy individuals with very low activity of ASA have been reported repeatedly [2-51, most of them relatives of MLD patients. However, unlike the MLD patients, it was demonstrated that intact cultured fibroblasts obtained from these individuals can catabolize the native sulfatide [3]. Therefore, this phenomenon is described as "pseudodeficiency" ciency is based on the existence of another allele at the ASA locus (ASAP) which codes for an enzyme with low in vitro activity, in addition to the normal allele (ASA+) and the allele for a deficient enzyme (ASA-); the pseudodeficient individuals are compound heterozygotes ASA-/ASAP (61. The existence of such an allelism was confirmed in the study of an isolate with a very high incidence of late infantile MLD [7]. A screening of healthy individuals from the said isolate uncovered 11 pseudodeficient adults (18-65 years old), none of whom had any clinical manifestation. The frequencies of both alleles ASA -and ASAP were calculated to be 0.115 and 0.137, respectively [8]. These observations, together with the increasing number of pseudodeficient individuals reported in the literature [9-131, suggest that the ASAP allele may be frequent in other populations as well. The polymorphism at the ASA locus predicts the existence of healthy individuals with the genotype ASA+/ASAp with low ASA activity probably in the MLD heterozygote range and individuals with the genotype ASAp/ASAp with lower ASA The most satisfactory explanation for the genetic basis of this pseudodefi-