The Apollo 11 astronauts were quarantined for 21 days after returning to Earth.
Neil Armstrong, Buzz Aldrin, and Michael Collins splashed down in the Pacific Ocean on July 24, 1969. After recovery, they entered a Mobile Quarantine Facility aboard the USS Hornet, where they remained separated from the public and most personnel. The quarantine was designed to prevent any hypothetical lunar microorganisms from reaching Earth, even though scientists considered such contamination unlikely.
The 21-day period was counted from the astronauts' departure from the lunar surface, so the confinement was not simply 21 full days inside the trailer. It included time aboard the spacecraft, the mobile facility on the recovery ship, and the Lunar Receiving Laboratory in Houston. NASA physician William Carpentier and engineer John Hirasaki were also quarantined with them.
No harmful lunar organisms were found. The quarantine ended on August 10, 1969, and similar procedures were used for Apollo 12 and Apollo 14. NASA discontinued the practice after Apollo 14, once the Moon was considered biologically sterile.