A beta-1,2-N-acetylglucosaminyltransferase which transfers N-acetylglucosamine from UDP-N-acetylglucosamine to the branched terminal mannosyl residues found in the complex type oligosaccharide units of N-linked glycoproteins was isolated from swine trachea mucosa. The enzyme was purified from microsomes after solubilization with 1% Triton X-100 and 1% Nonidet P-40, and it was also isolated from a soluble extract of this tissue. The enzyme was purified by chromatography on DEAE-cellulose, cellulose phosphate, and Sephacryl S300 columns and by affinity chromatography on a Sepharose 4B column containing covalently bound ovomucoid. The purified enzyme forms beta-1,2 bonds between N-acetylglucosamine and terminal-branched mannosyl residues of glycoproteins and glycopeptides. The enzyme has an absolute specificity for terminal branched mannosyl residues, and solute specificity for terminal branched mannosyl residues, and no activity is observed with mannose, p-nitrophenyl alpha-D-mannoside, p-nitrophenyl beta-O-mannoside, and glycopeptides which contain unbranched oligosaccharide chains terminating in free mannose. Evidence obtained by kinetic and structural analysis shows that the purified enzyme catalyzes the reactions shown in Scheme I.