Ovine submaxillary asialo-mucin was [14C]sialylated in vitro using a porcine liver cell-free preparation. The oligosaccharide chains were cleaved from the product glycoprotein by beta-elimination under reductive conditions, fractionated by gel filtration on Bio-Gel P-2 and characterized by thin-layer chromatography. The structure of the product chain was studied by periodate oxidation and analysis of the peeling products formed in the beta-elimination step. It appeared that [14C]-sialic acid had been introduced exclusively to the galactose residues of Gal beta(1 leads to 3)GalNAc disaccharide units occurring on the mucin as minor chains. No indication for a transfer to GalNAc residues on this glycoprotein was obtained. In agreement with this result sialyltransferase activities of porcine, rat, human and canine liver with Gal beta (1 leads to 3)GalNAc-protein acceptors were invariably much higher than those with ovine submaxillary asialo-mucin. When the asialo-mucin had been [14C]sialylated by an ovine submaxillary gland cell-free preparation analysis of the product oligosaccharide chain revealed the introduction of [14C]sialic acid to position C-6 on the GalNAc residues. The specificity of this transfer was reflected by the very high sialyltransferase activities of gland preparations with Gal beta (1 leads to 3)GalNAc-protein as well as GalNAc-protein acceptors. Mixed enzyme experiments indicated that the difference in liver and gland ovine submaxillary asialo-mucin sialyltransferase activities was not due to the presence of a specific inhibitor in the liver or an activator in the gland. It is concluded that porcine liver and likely liver of rat, man and dog contains a Gal beta (1 leads to 3)GalNAc-protein sialyltransferase, which is involved in the sialylation of O-glycosidically linked carbohydrate chains on serum glycoproteins. GalNAc-protein sialyltransferase activity, which richly occurs in ovine submaxillary gland, however, appears to be lacking from liver tissue.