Rat kidney plasma membrane preparations and suspensions of intact glomeruli were used to study glycoprotein: glycosyl transferase activities present on cell surfaces. Collagen: glucosyl, fetuin: galactosyl, and fetuin: sialic acid (sialíc acid is the generic term for N-acetylneuraminic acid, NANA) transferase activities were increased from 1.9- to 5.6-fold in membrane preparations compared to whole tissue homogenates. Considerable activity was found with suspensions of glomeruli though in each assay specific activity was substantially less than for glomeruli homogenized and sonicated in buffer containing the nonionic detergent Triton X-100. For both membranes and glomeruli, collagen: glucosyl and fetuin: galactosyl transferase activities were enhanced by addition of folic acid whereas sialic acid transferase activity was reduced