BI Special Agent Larry Welk has lots of secrets. Among them is one that's hard to conceal: His namesake, his great-uncle, also happens to be the King of Champagne Music.

"I was going through the checkout line at Hechinger's, and the saleswoman looked at my credit card and said, 'Oh, I know about you.' Very few people don't pick up on the connection," says Welk, whose full first name is Lawrence.

The champagne music Welk wasn't yet on television when the FBI Welk was born in 1953. But he made his debut a couple of years later, and Agent Welk remembers watching him. "I think he's a great entertainer, but the show was pretty schmaltzy," he says. "My grandfather -- his oldest brother -- was actually a better accordion player."

Agent Welk hasn't seen his great-uncle -- now 87, retired and living in California -- for about a decade. (The famous Welk didn't make it to the family reunion two years ago in tiny Strasburg, N.D., where his sod boyhood home is to be developed as a tourist attraction thanks to that $500,000 federal appropriation tucked in last year's deficit-reduction bud- get.) "The last time we spoke I was assigned to the New Haven FBI office and he was playing in Springfield. He was glad to see me," says Agent Welk, who now lives in Alexandria.

Over the years, Agent Welk has understandably collected a bit of Lawrence Welk memorabilia. There's the '50s mug with the "handle thing knocked off" that's signed "Musically Yours, Lawrence Welk." And a miniature car with "Champagne Music" on the side, from the days when the show was sponsored by Dodge Motor Co. And the "I love Lawrence Welk" pens, pencils and re- frigerator magnets from a Southern California retirement community. But the best is a pair of vintage, hand-painted ties that have his great-uncle's signature on the underside and a champagne glass with a conductor leading a sym- phony of bubbles on the front. "They're not really my style, and most of the bureau guys wouldn't appreciate them, but it's neat just to have them," the FBI man says. "In fact, I'm not musical at all --

I can just hum and play the kazoo."