The abilities of lectins to combine specifically with particular carbohydrate residues have made them valuable probes in the analysis of the surface components of intestinal epithelial cells. Fluorescence microscopy studies using FITC-labeled lectins have shown that the surfaces of these cells change as the cells move up the villi and that these changes are dependent upon the region of intestine in which the villi are located. The lectins have been used in radioassays developed to quantitate these cell surface components and have proven useful in monitoring the extraction and purification of these substances. Lectin affinity columns have been successfully used to isolate some of these components. Although lectins offer the advantage of specific recognition of relatively small defined carbohydrate units on complex molecules, they have the limitation of not being able to distinguish among different molecules bearing the same carbohydrate units. It is thus necessary to use the lectins in conjunction with other tools as one proceeds further in the analysis of the complex carbohydrates that constitute the cell surface.