Evidence is prisented which shows that the enzymatic defect in mannosidosis, the deficiency of alpha-mannosidase, is expressed in cultured skin fibroblasts from patients with mannosidosis. Additionally, by Cellogel electrophoresis, the enzyme can be separated into two major components (a heat stable component missing in mannosidosis fibroblasts with a pH optimum of 3.6-4.0 and a heat labile component present in mannosidosis fibroblasts with a pH optimum of 5.6-6.0). The specific activity and electrophoresis of alpha-mannosidase from cultured amniotic fluid cells are also shown to be similar to cultured skin fibroblasts.