Abstract: A sliding door assembly for a clean room for manufacturing precision parts, chemicals and special foods or for carrying out surgical procedures. The door assembly includes a pair of parallel vertically spaced guide shafts which are inclined at an angle to a horizontal and serve to guide the sliding door as it moves from an upper open position into a closed lower position. A door closing and opening actuator includes an actuator shaft extending between the guide shafts. A sealing arrangement of the door assembly becomes operational upon movement of the sliding door to the closed position.