Glycoprotein biosynthesis in the aortic wall has been approached by studying the mannosyl-transferase activity in the intimal microsomes. Studied on “ in vitro” acellular systems, the optimal pH is 5.8 and the optimal temperature is 27°C. This enzyme shows thermal inactivation which is compensated by an increased reaction rate, under the influence of its exposure to supra-optimal temperature. The affinity constant for the GDP-mannose substrate is 15 μ m. The enzyme is inhibited by nucleoside-diphosphates, GTP and its structural analogue, β-γ-methylene-GTP at concentration of 10 −3 to 10 −4 m. Mn 2+ and Mg 2+ ions are activators, and maximal activity is reached at a 5 × 10 −3 m concentrations of Mn 2+. Cemulsol, deoxycholate and Triton X-100 are inhibitors in strong concentrations. The same is true for cycloheximide and puromycin. The exposure of the enzyme to supra-optimal temperatures uncovers a varying behaviour as a function of low or high concentrations of substrate. There is an activating effect by the substrate in the presence of a high GDP-mannose concentration, after preincubation at supra-optimal temperature only.