We used immune electron microscopy to study the intracellular localization of sucrase-isomaltase, an intrinsic glycoprotein of the brush border membrane, to provide insight regarding the sites of its synthesis and intracellular processing and the mechanisms of its transfer to the brush border membrane. We identified the protein by postembedding staining with protein A-colloidal gold and by preembedding staining with peroxidase. The protein was found not only in the brush border membrane, but also in the endoplasmic reticulum including nuclear envelope, Golgi complex, smooth apical vesicles, and to a variable extent in the multivesicular bodies. Our findings are consistent with current concepts of biosynthesis of plasma membrane proteins, with synthesis, translocation, and initial glycosylation occurring at the membrane of endoplasmic reticulum and further processing occurring in the Golgi complex. The findings suggest the possibility that some intracellular degradation of sucrase-isomaltase occurs. Finally, our results appear to indicate that at least the final step of intracellular movement, transfer to the brush border membrane, is mediated by smooth apical membrane vesicles.