SANTA MONICA, CALIF., JUNE 6 -- Pop star Michael Jackson, who was hospitalized here late Sunday night complaining of chest pains, is suffering from a cartilage inflammation of the front part of the ribs and will be released "within the next 48 hours," Bob Jones, his personal publicist, said this afternoon.

Speaking in front of St. John's Hospital, Jones read a statement from Jackson's attending cardiologist, Mark Zatzkis, who said the 31-year-old singer is suffering from an inflammation called costochondritis, "which can be caused by overexertion and stress and is usually extremely painful."

This condition, Jones added, "is something that happens to football players a lot." He said Jackson has undergone "extensive laboratory and X-ray testing, including the heart and lungs," which revealed "no abnormalities."

After his release, Jackson will be an outpatient for seven to 10 days. The star's cardiologist told Jones that before Jackson "completely resumes full activities, a treadmill test is going to be performed." Asked if that was being administered to check his heart, Jones said, "I didn't ask him. He's a cardiologist, so presumably so."

Jones spoke in the quiet courtyard outside the main entrance to the hospital before a dozen journalists and about eight fans maintaining a vigil. Laid out on the grass were giant get-well banners brought by fans and signed by hundreds of well-wishers.

One, from the children of the St. John's Child Study Center, was scooped up by Jones after the press conference and taken in to Jackson, who has a much publicized fondness for and rapport with children.

On Tuesday youngsters from the Culver City YMCA arrived at the hospital unannounced to sing a song and give Jackson a banner. Although the hospital spokesman said security guards were momentarily unnerved, the children's banner was taken up to Jackson, who was sleeping at the time.

Only minutes after Jones finished his press conference, several of Jackson's siblings -- brothers Marlon, Randy and Jackie, and sister Rebbie -- arrived together to visit their brother. Smiling, they paused briefly at the entrance and were immediately mobbed by reporters asking questions.

"I haven't seen him yet," said Randy with a laugh. "He's a strong person," said Jackie. "I know he's gonna pull through."

Jackson's mother has been sighted at the hospital. His popular sister, singer Janet Jackson, was in Los Angeles last weekend but left Sunday morning for Tacoma, Wash., where she has concert dates. According to watchful fans, Jermaine Jackson spent Sunday night in the hospital with his brother and Michael Jackson's cook arrived this morning with shopping bags of goodies for him. Michael Jackson is said to be a vegetarian.

Spokesman Jones, however, denied that Jackson is having food brought in. "The hospital does not allow you to bring in your meals," he said.

Jackson was brought to the hospital late Sunday night by St. John's-affiliated plastic surgeon Stephen Hoefflin, a close friend of Jackson who may have performed some of the singer's well-publicized cosmetic surgery. But Jones told reporters, "The plastic surgeon was not the one who made the diagnosis."

Jackson has a ranch in the Santa Ynez Mountains, about two hours north of here, and also has a Los Angeles home 4 1/2 miles from the hospital, Jones said. Jackson had been working here on his newest album for several months when he was hospitalized. His pains Sunday night came while "he was dancing -- his favorite form of activity," according to Jones.

There's been speculation that Jackson has been suffering from stress over his long-overdue album as well as depression over the recent deaths of Sammy Davis Jr. and his grandmother, and perhaps of Ryan White, the young AIDS sufferer, as well.

"Stress is a very important factor," Jones said of Jackson's ailment. "I don't say he's worrying too much," he said, but "I would certainly like to see him rest more."

Asked if his recuperation would set Jackson back in his recording work, Jones said: "It's seven more days off. We're already eight months off schedule. Michael Jackson is one of the best artists around. I'm not concerned."

On Monday, the first day after Jackson was admitted, the hospital's public relations office handled 400 phone calls from fans and journalists. The hospital also has been inundated with balloons, flowers and cards.

The 551-bed Catholic hospital has grown accustomed to celebrity patients. Former president Reagan was in the hospital for tests this week. Elizabeth Taylor has been hospitalized for two months as she recovers from a near-fatal bout with pneumonia. Her room is on the fourth floor, as is Jackson's suite, and Jones confirmed that the two have spoken since he was admitted.

"It's getting to be routine," said John Wright, director of the hospital's Heart Institute and Cancer Center, who was standing outside observing the press conference.

Outside the hospital, a handful of fans have kept a vigil since they first heard the news on television Sunday, going home occasionally to sleep.

"Yesterday we went down to Hollywood Boulevard by his star {in the sidewalk} and got over a thousand signatures," said 20-year-old Shana Mangatal as she looked over a giant get-well banner made of construction paper.

"We're here to show Michael that his fans care," she said. "We grew up with his music."

"He's devoted all his years to us," added another fan, Sherry Atkins. On rock music station KROQ this morning, deejays Kevin Ryder and Bean Baxter took advantage of their morning guest, a hypnotist, to call a receptionist and then a nurse at St. John's Hospital to try to hypnotize them into putting Jackson on the line. Their attempts were futile.