Glycoprotein: galactosyl and glycoprotein: sialic acid transferase activity were measured in the serum of patients with renal insufficiency. Significant elevations in enzyme activity were found in all patients including those on regular hemodialysis. Galactosyl transferase activity was elevated in a group of renal transplant recipients with normal serum creatinine. Serum glycoproteins from renal failure serum have an altered carbohydrate composition, the major finding being a reduced galactose content. A smaller reduction in sialic acid content was also observed. The findings are discussed in terms of glycoprotein metabolism and the clinical setting of chronic renal failure.