Homogenates of cultured skin fibroblasts derived from patients with .alpha.-L-iduronidase-deficiency disorders (Hurler and Scheie syndromes) were capable of hydrolysing iduronosyl anhydro-[1-3H]mannitol 6-sulfate, although at considerably reduced rates compared with normal controls. The Vmax values of .alpha.-L-iduronidase from patients with Hurler or Scheie syndromes and from normal controls were 11, 12 and 833 pmol min-1 mg-1 of protein, respectively; the corresponding apparent Km values were 656, 50 and 53 .mu.mol/l, respectively. The .alpha.-L-iduronidases from normal and Scheie fibroblast homogenates exhibit pH optima at 3.6 and 4.1 and were competitively inhibited by both chloride and sulfate ions: Hurler .alpha.-L-iduronidase activity exhibited 1 pH optimum at 3.8 and was also inhibited by chloride and to a lesser extent by sulfate ions. The thermal stability of Hurler, Scheie and normal .alpha.-L-iduronidase activities at 55.degree. C gave half-lives of approximately 1.0, 2.5 and 1.0 h, respectively. Enzyme differences exist for these 2 clinically distinct phenotypes and provide biochemical evidence that the Hurler and Scheie syndromes result from different allelic mutations.