Various treatments carried out prior to the concanavalin A-horseradish perioxidase (HRP) method have been found to affect the staining and have permitted differentiation of three main classes of complex carbohydrates in the rat alimentary tract. Class I mucosubstances lose and class II and III paradoxically gain concanavalin A-horseradish peroxidase reactivity after periodate oxidation. Class II mucosubstances lose whereas class III retain or increase their reactivity with a reduction step interposed between oxidation and concanavalin A-horseradish peroxidase staining. Mucous neck cells, pyloric glands, Brunner's glands and mast cells exhibit strong class III staining, whereas other sites such as intestinal goblet and salivary gland acini differ widely in their type of staining. Liver glycogen stains like mucosubstances in an unstable subgroup of class III. The paradoxical increase in concanavalin A binding during oxidation correlates with the appearance of Schiff reactivity implicating oxidation of vicinal hydroxyls as the basis for the effect. The periodate-induced staining is therefore, thought to result from an oxidative disruption of linkages between vicinal hydroxyls of neighboring sugars and hydroxyls of mannose required for concanavalin A binding. Staining with the described concanavalin A-horseradish peroxidase variants appears to afford information concerning cytochemical distribution of mannose-rich glycoproteins as well as differences among these substances in the relation of mannose to neighboring sugars.