The beta-subunit of the gastric H,K-ATPase is the most abundant glycoprotein in the tubulovesicular compartment of the acid-secreting parietal cells. The oligosaccharides of the beta-subunit have been shown to contain fucose, N-acetylglucosamine, mannose, galactose, and N-acetylgalactosamine. Previous studies have shown that the rabbit beta-subunit is devoid of N-acetylneuraminic acid. Here we report the structural features of the N-linked oligosaccharides of the beta-subunit from rabbit H,K-ATPase. We used glycosidase digestions and analysis by high-pH anion-exchange chromatography with pulsed amperometric detection and matrix-assisted laser desorption/ionization mass spectrometry to analyze the peptide-N4-(N-acetyl-beta-D-glucosaminyl)asparagine amidase (PNGase F)- and endo-beta-N-acetylglucosaminidase H (Endo H)-released oligosaccharides. The studies showed that the oligosaccharides of the beta-subunit are a mixture of both oligomannosidic and lactosamine-type structures. The high-mannose structures were identified as Man5Man8GlcNAc2 species. A striking finding was that all the branches of the lactosamine-type structures were terminated with Galalpha-->Galbeta-->GlcNAc extensions. All of the lactosamine-type structures were found to be core fucosylated and some of them contained one to three lactosamine repeats. We propose that a part of the adaptation of the gastric beta-subunit to the acidic environment of the stomach is through providing acid-stable terminal residues on the oligosaccharides.