The tetrasaccharide fraction obtained by gel chromatography after treatment of commercially available heparin with nitrous acid was reduced with NaB3H4 and then hydrolysed with 2M trifluoracetic acid at 70 degrees for 3 days. By gel chromatography and electrophoresis, the 3H-labelled trisaccharide 1 bearing and unsubstituted 2-amino-2-deoxy-D-glucosyl group in the non-reducing position was obtained (18% from the 3H-labelled tetrasaccharide). By sequential, enzymic degradation, the structure alpha-D-GlcN-(1 leads to 4)-beta-D-GlcA-(1 leads to 4)-[1-3H]aManol was obtained for 1, which is a substrate for acetyl-CoA: 2-amino-2-deoxy-alpha-D-glucoside N-acetyltransferase, an enzyme that is deficient in the Sanfilippo C syndrome. In human-skin fibroblasts, the pH optimum of acetyl transfer onto 1 was between pH 5.5 and 7.0, and dependent on the buffer. An apparent Km for 1 of 0.14mM was found.