The ventricular epithelia of adult worker honey bees were investigated biochemically and ultrastructurally. The midgut tissues were shown to produce an endoprotease with trypsin-like activity. Enzyme activity was highest in the midgut tissues and the ectoperitrophic space of free-flying honey bees and of caged bees fed pollen. Lower levels of activity occurred in caged bees restricted to sucrose or fed artificial diets. The trypsin-like activity declined as the protein intake of the bees decreased with age. Ultrastructural studies revealed columnar cells in the posterior midgut engaged in the synthesis and release of membrane-bound vesicles. The apical cytoplasm of the epithelial cells in this region contains numerous electron dense vesicles which are released into the ectoperitrophic space of the midgut lumen. The microvilli in the crypts of this region are short, branching, and microvesiculate. Throughout the remainder of the midgut, the microvilli are profuse and elongate. The presence of the endogenously produced endoprotease and the regional variation in cell ultrastructure suggest that the honey bee may rely on countercurrent flow to distribute enzymes and nutrients efficiently throughout the midgut. Ultrastructural cytochemistry localized acid phosphatase and nonspecific esterase activity in primary and secondary lysosomes dispersed throughout the midgut tissues. Alkaline phosphatase activity was localized within large electron lucent microbodies that are present in all midgut columnar cells. The peroxisomal marker enzymes, catalase and L-α-hydroxy acid oxidase, were also localized in the same microbodies which previously had been described as holocrine secretory granules involved in dietary mineral regulation. Morphological and cytochemical assays suggest that the holocrine secretory granule arises from a microperoxisomal compartment involved in intermediary metabolism in the midgut of adult honey bees.