The mechanism of retaining glycosyltransferases is still poorly understood and the subject of current debate. Both double displacement and front side single displacement (SNi) mechanisms have been proposed. A "chemical rescue methodology" is here applied to a retaining α3-galactosyltransferase. Azide as exogenous nucleophile rescues the activity of the inactive E317A mutant to give β-d-galactosylazide. This result fits best with a double displacement mechanism in which Glu317 is the enzyme nucleophile involved in the formation of a glycosyl-enzyme intermediate.