Hexosaminyltransferase and glucuronyltransferase are well known for their role in the biosynthesis of proteoglycans. These two enzymes are characterized in rough and smooth membranes, obtained following subcellular fractionation of aortic media-intima. They require the presence of Mn2+ or Mg2+ for activity. The optimum concentration for these two cations is 5 mM. The optimum pH for hexosaminyltransferase and glucuronyltransferase is approximately 6.8 in Tris buffer, and their optimum temperature is 30 degrees C. Hexosaminyltransferase has an apparent Km of 0.27 nM. Glucuronyltransferase has an apparent Km of 0.21 nM. Uptake of labeled sugars by endogenous proteoglycans is inhibited by puromycin. Hexosaminyltransferase and glucuronyltransferase are present in both rough and smooth submicrosomal fractions. The different endogenous glycosaminoglycans are labeled during our incubation experiments. The greatest incorporation is noted for hyaluronic acid and heparan sulfate; the least is seen for chondroitin sulfate. The results obtained in vitro for incorporation of labeled precursors into endogenous proteoglycans are consistent with those observed during the study in vivo of the turnover of these macromolecules.