Two cloned fucosyltransferases, Fuc-t III and Fuc-t VI, are probed on a preparative scale with non-natural donor-substrates, in which the guanosine of the natural donor guanosine diphosphate-fucose is replaced by other purines, Surprisingly, the novel purine-diphosphate-fucoses (PDP-Fuc) are recognized by both enzymes as donor-substrates. Copyright (C) 1996 Elsevier Science Ltd