The N-glycans of recombinant proteins produced via the secretory pathway of cultured mammalian cells are often undersialylated, and insect cells lack sialytransferases. Undersialylated glycoproteins are rapidly cleared from the circulation, compromising the effect of pharmaceuticals. We show that incubation with living Saccharomyces cerevisiae cells expressing the catalytic ectodomain of rat liver α2,3-sialyltransferase (ST3N e ) in the porous cell wall resulted in sialylation of glycoproteins. The K m values of the yeast enzyme for several substrates were similar to those of recombinant ST3N e from insect cells and of authentic ST3N. The yeast strain provides an inexpensive self-perpetuating source of ST3N activity for glycan engineering of recombinant proteins.