A library of 11 UDP-N-acetylglucosamine analogs were rapidly screened for their activities as donors for the Neisseria meningitidis β1,3-N-acetylglucosaminyltransferase (LgtA) by direct on-chip reaction and detection with SAMDI-TOF mass spectrometry. Six of the analogs were active in this assay and were analyzed by SAMDI to characterize the kinetics toward LgtA. The analysis revealed that substitutions on C-2, C-4, and C-6 affect the activity of the donors, with bulky groups at these positions decreasing affinity of the donors for the enzyme, and also revealed that activity is strongly affected by the stereochemistry at C-3, but not C-4, of the donor. The study is also significant because it demonstrates that SAMDI can be used to both profile glycosyltransferase activities and to provide a quantitative assessment of enzyme activity. Copyright © 2011. Published by Elsevier Ltd.