WASHINGTON (AP) _ Gen. P.X. Kelley, the commandant of the Marine Corps, says he is doing all he can to encourage whistle-blowing in the service, going so far as to order a promotion for one sharp-eyed California corporal.

In an address to a group of defense industry executives two weeks ago, Kelley offered a story about computerized supply systems, the media and whistle-blowers.

Last year, Kelley recalled, the Marine Corps came close to paying $150 for an 85-cent hex nut for an obsolete jeep. But because of the corporal at Camp Pendleton, Calif., the order was quashed before it went out.

''Some people suggested that we should 'get' the whistle-blower,'' Kelley said. But the general said his response was to order an immediate, meritorious promotion for the corporal ''before the sun set in California.''

An account of Kelley's remarks was published by an industry newsletter, Tech Trends International, and confirmed Tuesday by the Marine Corps.

Kelley, in making his points to the National Security Industrial Association on Oct. 9, apparently exaggerated a bit in saying how much the hex nut would have cost the corps, referring to it as the $400 nut.

But he was clear with his basic point: ''We work hard (at uncovering) waste, fraud and abuse. And what happens when you go out and find these things? Immediately you're drawn and quartered by the press.

''And so it almost comes to the point where the young people say to me, 'Why should I find anything when all I'm going to do is get a black name publicly. Why should I even look?'''

According to Lt. Col. John Shotwell, a Marine spokesman, the corps didn't say much publicly about the incident last year. But on Aug. 7, 1985, Cpl. Lisa Branucci became a sergeant following Kelley's order.