Arylsulfatase A (ASA) is found to be deficient in healthy individuals (pseudo arylsulfatase A deficiency) who usually show in vitro ASA levels in the range of metachromatic leukodystrophy patients. The in vitro properties of ASA in pseudodeficiency were studied in cultured fibroblasts. The residual ASA activity showed apparent Km with the synthetic substrate (2.6 mM), pH optimum of activity (pH 5.0), and sensitivity to heat denaturation at 65 degrees C (T1/2, 10 min) similar to those found in controls. To test whether the low in vitro activity is a result of extreme sensitivity to the homogenization procedure, cells were disrupted by five different techniques, including rapid freezing and thawing, hand homogenization, ultrasonication, mild osmotic shock, and nitrogen cavitation; all yielded similar ASA ratio of the pseudodeficient to control. The use of antiproteases phenylmethylsulfonyl fluoride and leupeptin did not affect the residual ASA activity in the pseudodeficient line. These results imply that the ASA that is formed in this condition has properties similar to those of the normal hydrolase, so that even if it is synthesized in lower amounts, it is still sufficient to promote normal catabolism of sulfatide. Screening for ASA activity in lymphocyte extracts of a random sample of 250 individuals revealed 7 individuals with enzyme level in the MLD heterozygote range or lower. These individuals apparently represent homozygosity for pseudodeficiency (pd/pd). This implies that the frequency of the pseudodeficient allele is about 15% in the general population, leading to polymorphism of the ASA.