The subcellular localization of kallikrein was studied in the rat pancreas using the immunocytochemical protein A-gold technique. Kallikrein was found at the level of the rough endoplasmic reticulum (RER), Golgi cisternae, condensing vacuoles, and zymogen granules of the pancreatic acinar cells as well as in the acinar lumen. The effect of various tissue processings on the immunocytochemical labeling of kallikrein was evaluated using pancreatic tissue fixed in glutaraldehyde and embedded in Epon, Lowicryl K4M, or glycol methacrylate (GMA). Compared to the results obtained with Epon, Lowicryl allowed improved resolution and specificity in the immunocytochemical labeling, while GMA retained greater amounts of kallikrein antigenicity leading to a higher intensity in the labeling; since it also gave a good ultrastructural preservation, GMA appeared to be the superior embedding medium for the localization of kallikrein. The quantitative evaluation of the labeling obtained under the three embedding conditions showed the presence of an increasing concentration gradient along the RER-Golgi-granule secretory pathway, suggesting that, like other pancreatic exocrine enzymes, kallikrein is synthesized in the RER, processed through the Golgi apparatus, and packed in the zymogen granules before being released into the acinar lumen.