A federal judge, saying that 11 Navy recruits whose blood shows exposure to the AIDS virus are unlikely to win their suit to bar the Navy from discharging them, yesterday refused to issue an order blocking the discharge.

U.S. District Court Judge Louis F. Oberdorfer, who denied the recruits' request for a preliminary injunction, granted a 10-day stay of his ruling to permit them to appeal.

At a court hearing Friday, Navy lawyer Jeffrey Moon said that Navy Secretary John F. Lehman Jr. had decided to give the recruits honorable discharges. They originally had been offered general discharges.

Oberdorfer said the change "severely undercut" the recruits' claim that they would suffer irreparable harm if a preliminary injunction were not granted.

The plaintiffs reported for active duty in October and November 1985 and, within a week of taking their oaths, were given blood tests that indicated they had been exposed to the HTLV-III virus, which is believed to cause AIDS.

The Navy notified them early in December that they would be given "entry level separations" on the grounds of "erroneous enlistment."

The lawsuit does not challenge the military's overall screening policy for acquired immune deficiency syndrome. It applies only to a small number of Navy recruits who tested positive and had joined the service just before or during the start of blood screening for all military recruits last fall.

The 11 recruits claim that, because they already had been formally inducted into the service, they should be treated as "active duty" personnel who should be retained.

In his ruling, Oberdorfer said that it "is not irrational to treat recruits newly, if formally, inducted, in the same category as those not yet formally inducted."

"There is a rational difference," he added, "between the value to the Navy of personnel with six months' training and experience, and those who have been on active duty for only a few days."