Localization of proMBP and MBP in day-24 IL-5–induced UCC by immunoelectron microscopy using double labeling. ProMBP was localized with 15-nm colloidal gold particles and MBP with 30-nm gold particles. (a) Although proMBP labeling (small gold) is seen throughout the Golgi, it is concentrated predominantly in the large uncondensed granules (white arrow); only minimal MBP labeling (large gold) is evident in the large granules. In contrast, the small condensed granules show marked MBP labeling in the interior and minimal proMBP labeling at the periphery (black arrow). (b) At higher magnification, dense labeling for proMBP (small gold) and minimal labeling for MBP (large gold) is evident in a large uncondensed granule (white arrow). The black arrow identifies a small condensed granule that contains predominantly MBP, along with minimal proMBP at the periphery. (a) Original magnification ×11,000; (b) ×20,000.