Abstract Sulfamidase activity was determined in peripheral leukocytes from normal individuals and from members of four families with patients suffering from Sanfilippo's disease type A. [ N-sulfonate- 35S]Heparin was used as the substrate. The following results were obtained: 1. 1. In normal leukocytes a pH optimum of pH 4.5 and an apparent K m = 1 × 10 −4 mol N-sulfonyl groups/l was found. Upon standing at room temperature a two-fold increase of sulfamidase activity was found within two weeks. According to mixing experiments this was most likely due to the formation of an activator. 2. 2. After separation of leukocytes in granulocytes and a lymphocyte-rich fraction, sulfamidase activity was 6–15 times higher in lymphocytes than in granulocytes, depending on the pH of the incubation mixture. 3. 3. Under strictly standardized assay conditions a mean activity of 66.8 ± 24.0 pmol sulfate release/h/mg protein was found in normal leukocytes. No age- or sex-dependent changes of the activity could be observed. 4. 4. Very low levels between 0 and 3% of the normal mean sulfamidase activity were found in six patients with Sanfilippo's disease type A. 5. 5. In the leukocytes of their parents sulfamidase activity was reduced to 43 ± 16% of normal activity. Although there was an overlap between the activities of the groups of normal and obligate heterozygous individuals the probability of an individual being a carrier of the Sanfilippo A gene at a given sulfamidase activity could be calculated. On that basis, other family members were considered to be heterozygous carriers of the disease.