A medical problem with a crew member has delayed the first spacewalk of the Space shuttle Atlantis's mission to the International Space Station.
Nasa refused to elaborate on what was ailing German astronaut Hans Schlegel, but said it was not life-threatening.
Atlantis, which docked with the ISS on Saturday, was delivering Europe's Columbus science laboratory.
The lab is the first part of the ISS the European Space Agency will control. Installation will now start on Monday.
The Columbus laboratory cost about $2bn (£1bn) and has room for three researchers in fields ranging from crop breeding to the development of advanced alloys.
Nasa's website said that the medical issue would have "no impact to the overall mission objectives" and that the spacewalk on Monday would be conducted by Rex Walheim and Stan Love, who replaces Mr Schlegel.
Before docking, the crew guided Atlantis in a back-flip manoeuvre that allowed crew on the space station to photograph the shuttle's protective heat-resistant tiles.
Engineers on Earth will check the images for any possible damage that may have been done to the tiles during lift-off: a routine safety measure since the shuttle Columbia disintegrated on re-entering the Earth's atmosphere in 2003.
The 7m-long (24ft), 4.5m-wide (14ft), 12.8-tonne laboratory will be manoeuvred into position by the shuttle's robotic arm, and docked to the station's Harmony Node 2 connector.
Esa astronaut Leopold Eyharts will be staying on the station to commission Columbus, a process that should take a few weeks to complete.
Its installation will mean Esa becomes a full member of the orbital project.
Atlantis was launched from the Kennedy Space Center in Florida on Thursday, and is due to return to Earth now on 19 February, a day's extension to the originally planned 11-day mission.
Once the lab is in place, an intensive programme of research in weightless surroundings will begin.
The experiments will also help researchers better understand the physiological demands of long-duration spaceflight, something that will be important if humans are ever to colonise the Moon or travel to Mars.