IT'S BEEN a good eternity for the Isley Brothers. In September, their 40th album, the aptly titled "Eternal," entered the Billboard charts at No. 3 pop and No. 1 R&B, a remarkable achievement given that the first Isley Brothers single, the classic "Shout," charted in . . . 1959!

In fact, "Eternal's" first single, "Contagious," made the Isley Brothers the artists with the longest chart span on the Top 100 in Billboard history at 42 years. In pop music, that's as close to eternal as you can get.

Now, group founder Ronald Isley is about to embark on a second career as Mr. Biggs, movie star.

Mr. Biggs was created by Isley and R. Kelly in the latter's 1996 video for "Down Low (Nobody Has to Know)." In the steamy love triangle at the heart of both song and video, Mr. Biggs is a big-pimpin' Mafia-like don who seeks revenge on Kelly for getting "between the sheets" with his woman. Mr. Biggs has since appeared in the video for the Isleys' 1996 single "Floating on Your Love" and as Kelly Price's "uncle" in her hit, "Friend of Mine." Naturally, he's front and center in the Kelly-directed video for "Contagious" (with Chantee Moore as "the woman") and stars in the upcoming video for "Secret Lover."

"The hip-hop players in the business kind of look up to me because of my longevity in the industry," explains Ronald Isley, pointing out that, along with James Brown and George Clinton, the Isley Brothers are the most sampled act in hip-hop. "When I'd come around, it was 'You the man, you this, you that.' Mr. Biggs was sort of the unspoken title that was given to me, so we just put it on a record."

Now, Mr. Biggs will jump from the small screen to the Bigg one: Isley and R. Kelly recently finished writing the screenplay for a full-length feature that Kelly will direct, with both artists contributing songs. "We hope to have it out next year," says Isley, who also hopes to take Mr. Biggs to Broadway. "There'll be a lot to prove so we'll be getting up for it," he says, adding, "We have so much fun with it."

There's a slight downside to all this, says younger sibling Ernie Isley. Mr. Biggs has become synonymous with Ronald Isley, at times threatening to overshadow him. For instance, "Eternal" is credited to "The Isley Brothers featuring Ronald Isley a k a Mr. Biggs."

"There are a lot of young people who relate to my brother as Mr. Biggs and they're really not aware of 'Shout' or 'Twist and Shout' or 'It's Your Thing,' and all of the history that's involved in the Isley Brothers, and the musical changes that we participated in and thrived on, so that virtually everybody has copied or sampled our stuff and been very successful with it. But all they know is Mr. Biggs because that's when they came on board."

Those new fans might want to consult "It's Your Thing: The Story of the Isley Brothers," a three-CD career retrospective released two years ago. It traces the Isleys' evolution from their gospel roots and a false start as a doo-wop vocal ensemble to a group that helped shape '60s rock, '70s funk and '80s rap.

It was a 1959 concert at Washington's fabled Howard Theater that changed the course of the original Isleys, brothers Ronald, Rudolph and O'Kelly. While performing Jackie Wilson's "Lonely Teardrops," Ronald interjected the phrase "You know you make me want to shout," which apparently provoked a frenzied audience response. A rep from RCA, which had just signed the Isleys, noted the reaction and suggested their first single be built around that catchphrase. The resulting two-sided single, "Shout (Parts 1 and 2)," melded the call-and-response of traditional gospel with rock 'n' roll instrumentations and over-the-top vocal exuberance. Though it didn't reach the Top 40, "Shout" became one of rock's signature songs.

"A lot of the folks that knew my mother weren't too happy with the record because they thought the sound of it was blasphemy," recalls Ernie Isley, who was 7 when "Shout" was released. "They said 'You went into the church and you're talking about shouting!'

"I thought it was great. It sounded unlike anything on the radio, and [the original recording] still does."

The group's commercial breakthrough came in 1962 with the release of "Twist & Shout," though it would be overshadowed a year later by a copy-cat version from a fledgling British band called The Beatles. Ernie Isley, then 12, remembers the night in February of 1964 when the Beatles first appeared on "The Ed Sullivan Show."

"Over the next few days, there were meetings and my brothers were saying among themselves, 'Nothing is ever going to be like it was, everything has changed,' " Isley recalls. But the Isleys decided they were going to be all right because they had a promising young guitar player who'd sat in the Isley family's living room watching the Sullivan show, grinning the whole time. Kid named Jimmy James, which he later changed to Jimi Hendrix.

On the retrospective collection, you can hear Hendrix's first recordings on "Testify" and "Move Over and Let Me Dance." Ernie Isley, whose own distinctive guitar playing reshaped the band's sound in the '70s, points out "there's no other records out at that time with guitar playing like that." Next year may finally bring the release of another eight tracks featuring Hendrix's earliest guitar recordings. (Coincidentally, the Isley Brothers and Hendrix were inducted into the Rock and Roll Hall of Fame in the same year, 1992.)

The Isley Brothers began moving toward stardom with 1969's self-empowerment anthem, "It's Your Thing," and took off in the early '70s with the addition of younger brothers Ernie and Marvin and brother-in-law Chris Jasper. They transformed from a vocal trio to a self-contained band in which Ernie's fluid funk-rock guitar proved a perfect foil for Ronald's soulful vocals. They also changed their image, replacing the flashy suits of the '60s with the furs, silks and rhinestone outfits that defined the '70s.

The '80s brought a certain amount of turbulence -- O'Kelly Isley died of a heart attack in 1986, Rudolph retired and turned to ministry, and the younger Isleys and Jasper left to form their own group -- but the Isley Brothers continued, and Ernie and Marvin returned to the fold in 1990. Sustaining their hitmaking talents (the group has graced the R&B charts 50 times over 40 years), the Isley Brothers saw their catalogue tapped by a new generation of rappers in the '90s. Among the biggest beneficiaries: Notorious B.I.G.'s "Big Poppa" and Keith Murray's "The Most Beautifullest Thing in the World" (both sampling "Between the Sheets") and Ice Cube's "It Was a Good Day" (sampling "Footsteps in the Dark").

Then there was Michael Bolton. Earlier this year, the Isley Brothers finally celebrated a $7 million judgment against Bolton in one of the largest music copyright infringement cases ever. After a seven-year court battle, Bolton was ordered to pay up when the Supreme Court upheld a federal appeals court ruling that Bolton's 1991 pop hit, "Love Is a Wonderful Thing," infringed on the Isleys' 1966 recording of the same name.

While waiting to be paid his royalties, Ronald Isley filed for bankruptcy. In June, Wall Street financier David Pullman completed the sale of bonds backed by future royalties from the Isley Brothers' music catalogue. Pullman, who's made similar bond offerings with David Bowie, James Brown and Ashford & Simpson, didn't disclose the exact amount but said it was around the level of his previous music securitization deals, which range from $10 million to $55 million.

No one has benefited more from the Isley influence than R. Kelly. Come to think of it, R. Kelly might have gotten the Bolton treatment a few years back when his "Your Body's Callin' " bore a strong resemblance to the Isleys' "Groove With You," with Kelly's phrasing also similar to Ronald's.

"I had a few phone calls [from R. Kelly] in which he said 'I bit off a bit of this song or that song on 'Body's Callin',' and he was trying not to be sued," says Ronald Isley with a warm chuckle. "That's how we began to work together on 'Down Low' and from that day on it's like he's been a member of the family. He knows what the Brothers know and he's a genius in doing what he does."

Kelly wrote and co-produced three tracks on the Isley Brothers' 1996 album, "Mission to Please," as well as the new album's "Contagious." "Eternal" also features guest production by Raphael Saadiq and Jimmy Jam and Terry Lewis.

"We were going after what we felt would be an example of our best work and we hooked up with some folks who were fans but who were also talented in their own right," Ernie Isley says. "That's the alchemy we were looking for and it turned out very well."

Jam and Lewis have credited the Isley Brothers for early inspiration in becoming writers and producers. "We'd been wanting to work with them, and vice versa, for some time," says Ernie Isley. "When we finally hooked up with them, the first thing we did was an Isley Brothers seminar. They started asking about different songs, how they were put together, and we wound up going through a good deal of our catalogue. And we had a chance to ask them about some of the things that they'd done. It was like going back to old school -- a lot of fun."

THE ISLEY BROTHERS -- Appearing Friday and Saturday at Constitution Hall and Sunday at the Baltimore Arena. * To hear a free Sound Bite from the Isley Brothers, call Post-Haste at 202/334-9000 and press 8101. (Prince William residents, call 703/690-4110.)

Ronald (a k a Mr. Biggs), left, and Ernie Isley.