Human blood group A antigenicity of glycoproteins is retained on epon-embedded jejunum sections after glutaraldehyde fixation and osmium treatment. The intracellular location of molecules bearing these determinants was visualized in the four types of epithelial cells of A+ rabbit jejunum sections with immuno-colloidal gold labeling. The brush border membrane and in particular the glycocalyx of absorbing cells as well as the secretory granules of goblet and Paneth cells were heavily labeled. In enteroendocrine cells, the membrane of secretory granules and not their content was lightly labeled. The differential labeling of secretory or membrane bound glycoproteins is accompanied by different labels of the Golgi complex as expected if labeling of the Golgi saccules was due to the presence of glycoproteins in transit. In all cases the label is primarily concentrated in only half the cisternae on the trans side of the Golgi stacks. In absorbing cells, structures have been revealed in the terminal web that could be related to the brush border membrane and consequently implicated in its biogenesis. The fibrillar material of the glycocalyx appears as highly labeled tangled structures which apparently proceed from densely stained "carrier" vesicles arising from the Golgi apparatus. Vesicles fusing at the lower part of microvilli could result of integration of this material into the lightly labeled vesicles strictly found in the terminal web. These last vesicles could also contain newly synthesized brush border hydrolases.