Tumor cell surface sialic acid levels determine a number of important properties governing cellular interactions and cell-cell communication. Towards understanding the mechanism of regulation of sialic acid levels upon cellular transformation, we have studied the regulation of expression of β-galactoside α2, 6-sialyltransferase in a rat tumor, the Zajdela ascitic hepatoma. We demonstrate distinct differences in the regulation of expression of the enzyme in the tumor cells as compared to normal liver cells. The expression of sialyltransferase is regulated both at the transcriptional and post-transcriptional level in a tissue-specific manner. © 1993.