Sucrase-isomaltase complex is expressed in human small intestine throughout gestation and in the large intestine between 12 and 30 wk. The molecular form of the enzyme was studied in the brush-border membrane fractions by the immunoblotting method. Before 30 wk of gestation, the enzyme is present only as the high molecular weight prosucrase-isomaltase, while from 30 wk until birth the two subunits are also present. The fetal enzyme, as its proform and as its two subunits, has a faster mobility in sodium-dodecylsulfate polyacrylamide gel electrophoresis, than the adult enzyme (removal of sialic acid residues from fetal enzymes emphasizes this difference). The colonic and the small intestinal fetal enzymes are identical.