The Apollo-Soyuz was a test project to see if it was possible for Russia and United States to dock and have future joint missions.
On July 15 to the 24 of 1975 the Apollo crew, Thomas Stafford, Vance Brand, and Donald Slayton joined with the Soyuz Crew, Valeriy Kubasov and Alexei Leonov.  Together the crews completed expirements and docking maneuvers for two days.  After that the two ships went their seperate ways.
Information: ksc.nasa.gov