A sialyltransferase having the following physico-chemical properties; (1) Activity: transfers sialic acid from a sialic acid donor selectively to a 3-hydroxyl group of a galactose residue contained in lactosylceramide as a sialic acid acceptor to produce ganglioside G.sub.M3; (2) Optimal reaction pH: 6.0 to 7.0; and (3) Inhibition and activation: the activity increases at least 1.5 times with 10 mm of Mn.sup.2+ as compared with the case in the absence thereof.