A 22nd-century programme that posted physicians to the worlds of other species to share medical knowledge, bringing the Denobulan doctor Phlox to Earth and sending the human Jeremy Lucas to Denobula.
The Interspecies Medical Exchange is the scheme under which Doctor Phlox is working in San Francisco when the Klingon courier Klaang is shot in Broken Bow, which leads Jonathan Archer to recruit him as chief medical officer of Enterprise NX-01.
Phlox's counterpart is Doctor Jeremy Lucas, a human physician stationed on Denobula. The two exchange letters, a correspondence that frames the episodes Dear Doctor and Doctor's Orders, and Lucas later appears in person as a hostage at Cold Station 12.
The exchange also holds conferences. In Stigma Phlox uses one on Dekendi III to ask Vulcan physicians discreetly about Pa'nar Syndrome on T'Pol's behalf, which exposes her condition to the Vulcan medical establishment.