Microsomes prepared from bovine submandibular glands incubated with radioactive AcCoA incorporated acid-insoluble radioactivity, which was dependent on time, and the concentrations of AcCoA and proteins, and was inhibited by CoA in a concentration-dependent manner. Under the conditions used, the apparent Km for AcCoA was 1.63 microM with a Vmax of 21.9 pmol/mg protein.min. The radioactivity incorporated was mainly due to the O-acetylation of glycosidically bound Neu5Ac. The primary attachment site of O-acetyl groups was exclusively the hydroxyl at C-7 of Neu5Ac, the presence of an AcCoA:Neu5Ac 7-O-acetyltransferase thus being demonstrated. After longer incubation 9-O-acetylated Neu5Ac also appeared, suggesting the migration of an ester group from C-7 to C-9. This isomerisation was inhibited by heat-inactivation of the microsomal protein, enzymatic isomerisation by a "migrase" thus being suggested. Data are presented which lead to the assumption that this 7-O-acetylation involves at least two reactions: the transport by a translocase of acetyl groups from AcCoA from the cytosol across the Golgi membrane, followed by the enzymatic transfer of these acetyl groups onto sialic acids in the Golgi lumen.