The treatment of H.Ep.2 cells with 4 mM aryl-N-acetyl-alpha-galactosaminide, an inhibitor of the elongation of O-linked saccharides of glycoproteins, resulted in a 70-80% decrease in the incorporation of [3H]glucosamine into epitectin (a MUC 1 mucin-type glycoprotein). Gel electrophoresis of the partially glycosylated epitectin immunoprecipitated from the extracts of treated cells showed two bands of apparent mol. wts 420 and 460 kDa on SDS-PAGE, compared to bands of 350 and 390 kDa for the native epitectin. Analysis of the saccharides released by beta-elimination from the partially glycosylated epitectin showed the presence of substantial levels of beta 1-->3 galactosylated and sialylated saccharides. The data indicate that the inhibition of elongation of Ser/Thr-linked N-acetyl-galactosamine in epitectin by aryl-N-acetyl-alpha-galactosaminides is only partial.