A sterile containment welding device for plastic tubes comprises a pair of side by side spaced tube holders. Each of the tube holders includes a clamp base and a clamp lid. A pocket is provided in each clamp base extending completely across the clamp base so that a plastic tube could be inserted into the pocket and clamped in the holder when its clamp lid is moved to a closed position. Pockets of the side by side tube holders are alignable with each other to permit a plastic tube in one of the holders to be welded to the plastic tube in the other of the holders. A tube locating member is movably mounted at the end of each pocket in the space between the holders. The tube locating member has a wall disposed in the space beyond the pocket for positioning its plastic tube in the proper position for the later welding step when its tube is moved into contact with the walls. An inactivating member is mounted to each clamp lid to contact the tube locating member and move it out of contact with its plastic tube upon the closing of the lid so that the tubes are exposed to be heated and welded together.