Gallbladder mucin is a densely glycosylated macro-molecule that promotes cholesterol gallstone formation in experimental animals and in humans. Bovine gallbladder mucin structure was studied after chemical deglycosylation by treatment with anhydrous hydrogen fluoride at 23 degrees C for 3 hours. Deglycosylated mucin contained less than 5% of the amino sugar and neutral hexose content of native mucin. Electrophoretic and molecular sieve chromatographic analyses indicated that significant cleavage of the mucin polypeptide core had occurred during deglycosylation. Deglycosylated mucin was separated into three major fractions by reverse-phase chromatography, one of which was enriched with respect to threonine and proline. Tryptic peptides prepared from this fraction were purified by molecular sieve and reverse-phase chromatography, and the amino acid sequences (8-20 residues) of the four principal tryptic peptides were determined. These peptides contained 65%-75% threonine and proline residues and demonstrated 80%-100% sequence similarity. These data provide the first information on the primary structure of gallbladder mucin and suggest that repeating amino acid sequences occur in this protein. Comparison of gallbladder mucin peptide structure with the consensus repeat sequence of human intestinal mucin showed approximately 60% sequence similarity. It was concluded that mammalian gastrointestinal mucins may be derived from a common ancestral gene.