A significant proportion of patients affected with fucosidosis have Italian ancestors. We assayed for alpha-fucosidase activity purified mononuclear cells and/or leukocytes obtained from 64 members of two large pedigrees from Calabria, in which seven children had been diagnosed as affected with fucosidosis. Of these 64 individuals, 22 were diagnosed as carriers, while the values for the remainder were within normal limits, indicating a clear bimodal distribution among individuals at risk. These data confirm that carrier detection for fucosidosis requires the measurement of alpha-L-fucosidase activity in purified mononuclear cells from peripheral blood. In addition, this program has made prenatal identification of an affected fetus possible.