The size of isolated gastric mucin is substantially larger if proteolysis is rigorously excluded during the isolation procedure by inclusion of proteinase inhibitors with or without guanidinium chloride. Pig gastric mucin isolated in 0.2 M NaCl (without proteinase inhibitors) so25,w, 33S, Mr approximately 2 x 10(6) has consistent physical and chemical properties, retains the gel-forming properties of the native mucus secretion and is readily susceptible to proteolysis although it has undergone proteolytic "nicking" during isolation. Gastric mucins isolated in proteinase inhibitors with guanidinium chloride are of larger size so25,w, 41-110S but precipitate from solution at gel-forming concentrations. Reduction of all these gastric mucin preparations with either 0.2 M mercaptoethanol or 10 mM dithiothreitol produced subunits of size in the region of Mr approximately 5 x 10(5). Gel filtration studies also showed that reduced mucin subunits prepared from gastric mucins isolated in guanidinium chloride could reassociate to form larger subunits of size approximately Mr approximately 2 x 10(6). The dependence of gastric mucin size on the isolation procedure is discussed.