Binding sites for concanvalin A (Con A), Ricinus communis I agglutinin (RCA I), and Helix pomatia lectin (HPA) were localized in the Golgi apparatus of rat small intestinal absorptive cells. A preembedment technique, a modification of the one originally used by Bernhard and Avrameas (Exp Cell Res 64:232, 1971), was employed, with horse-radish peroxidase being used for cytochemical visualization. Incubations were performed on 10 microns thick cryosections of duodenal segments that were fixed in a mixture of 4% formaldehyde and 0.5% glutaraldehyde; fixation was preceded by a 2-min rinse in 0.1 M sodium cacodylate and followed by storage in the same buffer for up to 7 days. Incubation with Con A, which binds preferably to alpha-D-mannose and alpha-D-glucose residues, caused intense reaction of the dilated cisternae of the cis Golgi side; staining was variable in intermediate and trans cisternae. RCA I, recognizing beta-D-galactose residues, could only be demonstrated in intermediate cisternae. Reaction for HPA, which indicates alpha-N-acetyl-D-galactosamine residues, stained intensely 1 to 2 cisternae of the cis Golgi side, as well as being localized in the peripheral regions of the cisternae of the intermediate compartment of the stacks. Deposits of reaction product covered the luminal surface of the cisternal membranes, but usually left the lumen itself, as well as lipid particles, devoid of reaction. The differences in Con A, RCA I, and HPA reactivity between cis, intermediate, and trans cisternae suggest compositional and structural differences of the carbohydrates in the respective compartments; they may reflect conversion processes that are known to occur in the oligosaccharide side chains of glycoconjugates at the Golgi complex level.