An adapter for use on an anesthesia machine having of three or more vaporizer units removably mounted thereon by an interlock/exclusion system. The interlock/exclusion system includes plural vertically oriented pins that are coupled together and are operative to move upward to prevent opening of any vaporizer units if one is open. One of the vaporizer units includes a horizontally oriented pin which moves outward when that unit is opened. The adaptor is arranged to mount that vaporizer on the machine and to couple the horizontally oriented pin to an associated vertically oriented pin of the interlock/exclusion system.