Muscarinic acetylcholine receptors in isolated rat pancreatic acinar cells have an apparent Mr of 88 000, which could be decreased to 46 000 by papain, as deduced by covalent binding of the specific alkylating agent [3H]propylbenzilylcholine mustard. Muscarinic receptors on papain-treated acinar cells retained the antagonist-binding site and both high- and low-affinity binding sites for the cholinergic agonist carbachol. Similar results were observed in studies with rat parotid acinar cells, although the receptors in both control and papain-treated cells were each 10 000-15 000 Da smaller than in pancreas. Additionally, muscarinic receptors in papain-treated pancreatic acinar cells retained the ability to mediate carbachol stimulation of digestive-enzyme secretion. These results demonstrate that the characteristic binding properties of muscarinic receptors for both agonists and antagonists as well as their ability to translate agonist occupancy into a physiological response are not altered by proteolytic cleavage.