Describes measurements made in 1979 on the refectory of S. Maria delle Grazie, Milan: the degree of humidity in the walls was controlled as was the temperature of the pavements and walls, the air circulation, and the constant temperature. Noting that the humidity of the room has been a problem since the early 16th century, the authors suggest an ideal ambient of humidity under 55 percent with a temperature of not more than 25°C. Among the most important pieces of equipment needed now is a filtered ventilation system.