Columbia's astronauts drew more blood and tested an animal isolation chamber Friday while ground engineers checked the Discovery to see if an emergency spacewalk repair can be averted.

Two of the astronauts may have to step into space as early as Sunday if the tests on Discovery show a loose seal will prevent Columbia's cargo bay doors from closing tightly. Crewmembers James Bagian and Tamara Jernigan are trained for such a job.The two massive doors covering Columbia's cargo bay must shut tightly when the spaceship begins the fiery plunge through the atmosphere at the end of the nine-day flight June 14. Otherwise, a door edge could be exposed to the heat of re-entry and the shuttle could burn up.

If the doors close properly but the loose seal causes a gap, the seven crew members would be safe but Columbia could be damaged by hot gases seeping into the cargo bay, NASA said.

A special team of engineers and a spacewalking veteran flew to Kennedy Space Center on Friday to duplicate the problem on Discovery. The test involved snipping the seal in the same place where it is broken on Columbia, then closing a cargo bay door to see if the strip of reinforced rubber prevents a tight fit.

Kathryn Sullivan, the first and only American woman to walk in space, was standing by with space gloves to rehearse a possible repair job in Discovery's hangar. The testing was expected to continue through the night.

Engineering analysis from other tests showed "every indication that we have little requirement to do anything other than . . . close the doors and come home," said NASA flight director Randy Stone.

Depending on how tests on Discovery go, he said a spacewalk might not be needed, but he wouldn't rule out the possibility of one.

In a message faxed up to the crew Friday, Mission Control said the engineering team was considering all options "to assure ourselves and you" that the doors will work properly.