Using 4-methylumbelliferyl-.alpha.-L-iduronide as a substrate, .alpha.-L-iduronidase activity was measured in leukocytes and in lymphoblastoid cells obtained from patients with .alpha.-L-iduronidase deficiency [Hurler syndrome and Scheie syndrome] and from obligate heterozygotes for this disease. There was complete discrimination between .alpha.-L-iduronidase activities measured using 4-methylumbelliferyl-.alpha.-L-iduronide in leukocytes and in lymphoblastoid cells from the patients and controls. Overlap was observed between values of the activity in obligate heterozygotes and those in controls. 4-Methylumbelliferyl-.alpha.-L-iduronide is superior to phenyl-.alpha.-L-iduronide for determination of .alpha.-L-iduronidase activity because of greater sensitivity, easier assay procedure and shorter incubation period.