Jon Hill, who took over as White House chef only four months ago, has left the post to return to the hotel industry. Hill announced his resignation on Wednesday and left the job Thursday. No reason for the action was given by either the White House or Hill, who could not be reached for comment.

There had been unconfirmed reports that Hill was not working out and that the food at state dinners did not measure up to past standards. But Nancy Reagan's press secretary Elaine Crispen, asked for the first lady's opinion of Hill, said, "I know of no complaint that she had." Crispen added that she thought Hill "did a commendable job with the Gorbachev dinner and Christmas events we had."

Assistant chef Hans Raffert will serve as acting executive chef until a replacement for Hill can be found, and Frank Ruta, who was invited back to the Executive Mansion in December to his former job as a chef to the first family, will instead become assistant chef.

Crispen said Hill did not inform her of his future employment plans. There as no mention of "returning to any particular hotel."

Unlike other such announcements, the White House press release disclosing Hill's departure did not include a statement of appreciation from Mrs. Reagan. Nixon at 75 Former president Richard Nixon discouraged plans to celebrate his 75th birthday today with friends and former colleagues from Washington. Instead he'll spend the day at home in Saddle River, N.J., with family. The White House said President Reagan has acknowledged Nixon's birthday in the past and will do so this time. The form of acknowledgment, however, was not disclosed.

The 37th president's birthday dinner will be Italian food prepared by his bodyguard Mike Endicott, a gourmet cook. Nixon gave up his Secret Service protection in March 1985 and pays Endicott himself. "Groups of friends and colleagues both in Washington and New York approached him at various times and wanted to throw a party," said Nixon aide John Taylor. "He thanked them and said he preferred to keep his birthday a private family matter."

Meanwhile, in Florida, vendors are reportedly selling T-shirts emblazoned, "Tanned, Rested and Ready: Nixon in '88!"

Max Robinson's Condition Improves Former ABC News anchor Max Robinson's condition has been upgraded from critical to serious by St. Francis Hospital near Chicago, where he is being treated for an undisclosed ailment. Robinson, 48, has been hospitalized since Dec. 4. His release, however, is not expected soon, said a nursing supervisor.

Opera at a Bargain Never underestimate the effect of a little snow on Washington. The Washington Opera, fearing light attendance at today's matinee performance of "L'Italiana in Algeri" at the Kennedy Center, is selling $42.50 tickets for $10 apiece. More Questions for the Bakkers A PTL trustee says he plans to investigate reports that Jim and Tammy Bakker have had extensive plastic surgery at the TV ministry's expense.

David Clark noted an IRS report that $4,069 was paid to a cosmetic surgery center on behalf of Tammy Bakker. Past and present PTL employees have said Tammy Bakker had fat suctioned from her thighs, stomach and hips, plus a tummy tuck and breast enlargements, and that her husband went in for a face lift.

Clark says he will sue the couple to recover the money if the reports are true.

Cicciolina's Partial Apology Italian porn star and Parliament member Ilona Staller is in trouble with fellow Radical Party members who've grown impatient and angry with her antics, including baring her breasts in foreign nations and talking up free love. Her critics spoke out this week when Cicciolina ("Little Fleshy One"), as she's known professionally, announced that she'd run for party secretary.

Staller, 36, conceded that she has made mistakes and asked fellow members to teach her how to be a competent politician. However, she later announced plans to visit Argentina, where she hopes to set up "love parks" where couples could indulge their sexual fantasies freely and where telescopes would be given to voyeurs.

Jack Klugman's Hollywood Star Veteran actor Jack Klugman was honored Thursday with the 1,863rd star on the Hollywood Walk of Fame. Several hundred people were on hand to hear Klugman say, "It's nice to see my name in cement. Where I come from, there are whole bodies in cement."