Hoist a pint, sing along and beware the hoary jokes: That's the formula of Interact Theatre Company's ongoing "Bull and Bush" franchise at Arena Stage's Old Vat Theater. The current edition, another "Christmas at the Old Bull and Bush," finds the company in warm holiday form, mixing the naughty with the nice and finding a sentimental ending.

The basic "Bull and Bush" idea, cooked up by director Catherine Flye, is to re-create the atmosphere of an old-time English music hall (the Old Bull and Bush being one of London's most revered public houses for more than three centuries). Set and lighting designer Carl F. Gudenius effectively turns the Old Vat into the cozy interior of an English pub, complete with two working bars offering beer, wine and hot food on either side of the small stage before the show and at intermission. The authentic pub grub and casual atmosphere are no small part of the "Bull and Bush" charm.

As for the entertainment--well, it's a lot of ham and cheese without apologies. There is a running gag about what the young lady said to the sailor, with each punch line augmented by a happy leer at the audience from Timothy B. King as young Charlie Potts. There's Margie Tompros as Miss Bessie Bellwood, Empress of Noise, who enters singing in a wide, wide variety of animal voices (the sheep gets the biggest laugh--great vibrato). There's a bit where the lush at the bar (Albert Coia) asks what's in his bowl.

"It's bean soup, sir," explains Brian O'Connor as the Chairman (a k a the emcee).

"I don't want to know wha' it's been," Coia complains. "I want to know wha' it is now!"

And so on. The company's got a million of 'em; the joke book they're working from is older than Milton Berle. But Flye believes that the spirit of the thing is what makes it fun, and she gets a light, unpretentious performance from her cast that generally proves her right.

The show is full of throwaway comic songs and audience participation numbers, but baritone Bob McDonald's robust rendition of "The Road to Mandalay" puts you on alert that the music might actually be pretty good. Every so often, McDonald steps forward with something lovely or a group of six or seven sings in fine close harmony. Credit here goes to music director Larry Edward Vote as well as to accompanist Lisa Gibbs Smith, who rises from the piano now and again to join the small chorus.

The comic keepers include much of Coia's work. With his terrible brown plaid suit (by costume designer Rosemary Pardee) and tipsy, gently bawdy manner, he's a harmless little barfly, the shy kid trying out a dirty joke in the boys' room. But the evening's topper is probably "Wanting You," McDonald's love duet with a grim-faced chicken that is given voice by Tompros, hidden behind a bar. It's a pure Muppet moment.

Some of the material seems new, like the silly reindeer ballet that opens Act 2 to the tune of Tchaikovsky's "Dance of the Sugar Plum Fairy." But much of the rest has been seen before. It couldn't be a "Bull and Bush" without "Knees Up, Mother Brown," for instance, or "Danny Boy," and this Christmas show continues to derive its wee bit o' depth from W.R.M. Percy's "Letter From the Trenches," a World War I missive that describes an impromptu peace between the English and the Germans at the front.

Of course, come the holidays, familiar material blankets us like snow. These days, it often seems that Christmas entertainment is partly about institution-building (a k a "tradition"). As such things go, this one's not bad.

Christmas at the Old Bull and Bush. Directed and devised by Catherine Flye. Choreography, Linda Garner Miller; wig design, Ariel Butner. With Rebecca Davis and Karen Paone. Through Jan. 9 at Arena Stage's Old Vat Theater. Call 703-218-6500.