Glycoprotein: sialic acid transferase activity wasstudied in kidney tissue utilizing as exogenous acceptors desialized fetuin and desialized Tamm-Horsfall urinary glycoprotein. Activities were determined in cortex, medulla, and glomeruli. The effect of pH, divalent cations, and different protein acceptors was noted. The identity of the radioactive product formed was shown to be [<sup>14</sup>C]-N-acetylneuraminic acid labeled glycoprotein by hydrolysis and isolation of the radioactivity. This enzyme activity may prove useful in the study of renal pathology.