When the intracellular transit of 3H-labeled (pro)-insulin polypeptides is perturbed by monensin in the pancreatic B-cell, proinsulin conversion is impaired and the radioactive peptides accumulate in a clathrin-coated membrane compartment related to the Golgi apparatus. Clathrin was demonstrated by immunocytochemistry using the postembedding protein A-gold technique. The coated compartment, which is dilated by monensin, comprises Golgi cisternae with condensing secretory material and newly formed secretory granules; under monensin block, the noncoated (storage) secretory granules do not become significantly labeled. These data suggest that an unperturbed passage through a Golgi-related, clathrin-coated membrane compartment which subsequently matures into noncoated secretory granules is needed for the normal processing of (pro)insulin polypeptides.