We previously isolated an unusual hamster cell mutant (/d/D) that does not express LDL receptor activity unless it is cocultivated with other cells or grown in high concentrations of serum. We now show that IdlD cells are deficient in the enzyme UDP-galactose and UDP-N-acetylgalactosamine(GalNAc) 4-epimerase. When/d/D cells ate grown in glucose-based media, they cannot synthesize enough UDP-galactose and UDP-GalNAc to allow normal synthesis of glycollpids and glycoproteins. The 4-epimerase deficiency accounts for all glycosylation defects previously observed in/d/D cells, including production of abnormal LDL receptors. All abnormal phenotypes of/d/D cells can be fully corrected by exogenous galactose and GalNAc. The separate effects of these sugars on LDL receptor activity suggest that O-linked carbohydrate chains are crucial for receptor stability./d/D cells may be useful for structural and functional studies of many proteins, proteoglycans, and glycolipids containing galactose or GalNAc.