THE WELCOME MAT is out at Johnny Boy's, the best little rib house in Maryland. It is no more than a roadside splotch, a drooping pile of makeshift signs and peeling paint perched on cinderblock. Then, without warning, the olfactories come to attention and salute.

What await you are prize-winning ribs, golden brown and laden with the smoke of oak and hickory wood. The wondrous cooking instrument that makes all this possible is an open pit, an outdoor structure of cement block with a grill over an open fire.

Most area jurisdictions prohibit or closely regulate open-pit cooking, although some allow it for short-term fairs and church functions. In Charles County, where Johnny Boy's sits on the southern fringe of La Plata, this mode of cooking is legal.

Maybe it's the informality of the country, teamed up with the aroma of the fire, that invokes fanaticism on the part of serious rib-seekers who hunker down here. Or maybe it's the sauce.

Probably, it's both.

We will tell you this much: there's no use twisting the arm of Sophie Katsouros, mother of the one, the only Johnny Boy, for the recipe. She's a spitfire of a septuagenarian, and the secret is known only to her, Johnny Boy and God.

From their screened hut with adjoining picnic tables, mother and son peddle more than 1,200 pounds of ribs every week in the summer. More than a few have Frank Herzog's name on them.

The gregarious WJLA sports anchor and radio voice of the Washington Redskins, family in tow, makes regular pit stops at Johnny Boy's. The mere mention of J.B.'s brings a lilt to Herzog's voice, a smile to his face.

"They're the best ribs I've ever had in my life," he gushes. "And I like the shredded pork and shredded beef even better."

Herzog discovered rib heaven by accident one night. "I went down to that drag strip at Budds Creek to do a story. Coming home, we were starving and we went by this place where you could see the barbecue smoke. We pulled over and I said, 'Look at this, it's such a dumpy little place. This has got to be the best barbecue place in the world because look at how bad the place looks.' "

Johnny Katsouros, 47, has been known to whisk a load of his delicacies to the announcers' booth in RFK Stadium on any given Sunday.

"He shocked us," laughs Herzog. "Sonny Jurgensen and Sam Huff love ribs. The three of us sat there trying to do a football game and eat ribs during commercials. It was hilarious."

Herzog travels Route 301 to avoid the eternal construction on I-95. "We like 301, it's such a different drive from the Interstate, which is nothing but a succession of cloverleafs and road signs. There's some character here, you know? If you want to stop and get a fresh apple on the way, or a cold can of beer, you know you can do it on 301."

Johnny Boy doesn't have much time for thinking about concrete configurations and high-priced signs. He has a business to look after, and spends his hours listening to the sizzle and snap on his shopworn grill.

He's confident that his message is carried to would-be rib eaters in the smoky scent on the wind. "People don't have to read the sign to know that it's a barbecue," he reasons. "If I was to build a fancy cinderblock place and make it so people can come and sit down, well, you lose somethin', you know?" GETTING THERE -- To the southbound motorist, Johnny Boy's is on the left side of Route 301,about one mile south of La Plata in Bel Alton. No phone. Hours of operation are very relaxed. In colder weather, it is open Wednesday though Sunday, 11 a.m. to 6 or 7 p.m. In warmer weather, open every day but Tuesday, 11 a.m. to 10 p.m. Sliced or shredded beef or pork, $2.50 to $2.75; rib sandwich, $3.75; slab of ribs, $12.50; half-slab $7.50.