Skin fibroblasts from a patient with Fabry's disease showed deficient activity of alpha-galactosidase. Fibroblasts from his mother and sister had two distinct clonal populations, one with enzymatic activity and the other enzyme deficient. This provides evidence of genetic inactivation at the alpha-galactosidase locus and makes possible the detection of carriers of Fabry's disease even when the enzymatic activity in their leukocytes and uncloned fibroblasts is within the range of controls.