1. Jejunal biopsy specimens from three children with congenital sucrase—isomaltase deficiency were assayed for disaccharidase activity and were subjected to analytical subcellular fractionation with enzymic microanalysis.
2. By use of the highly sensitive fluorigenic modification of the disaccharidase assay, brush-border sucrase and isomaltase activities were depressed but nevertheless detectable in each child.
3. Apart from the expected decrease in brush-border α-glucosidase activity, the other enterocyte marker-enzyme activities were normal.
4. There were no abnormalities in the enterocytes of any child on analytical subcellular fractionation or on electron microscopy.