Immunoreactive actin molecules have been localized in pancreatic acinar cells by the protein A-gold technique. The labeling was found at the level of the filamentous cell web, and in close association with the Golgi cisternae, condensing vacuoles, and zymogen granules delimiting membranes as well as with the plasma membrane. A weak labeling was also present over the dense content of the zymogen granules. The association of actin with different membranes implicates that contractile proteins might constitute structural membrane proteins and, thus, might play an important role in protein secretion.