A federal jury here acquitted the assistant manager of the Palm Restaurant yesterday of charges that he helped arrange the $1,650 purchase of one ounce of cocaine by an undercover drug agent at the expensive restaurant frequented by lawyers, journalists and executives.

Thomas James (Tommy) Jacomo, the assistant manager, is the first of 20 persons to come to trial on charges stemming from a massive investigation by the Drug Enforcement Administration, the D.C. Police Department and the FBI into cocaine activity in downtown Washington restaurants.

Assistant U.S. Attorney Barry Leibowitz of the major crimes division described Jacomo during the trial as the "maitre d' of cocaine" and financier of cocaine purchases and sales at the Palm.

Jacomo's attorney, Robert Watkins of the firm of Williams and Connolly, acknowledged that his client had used cocaine and had dealt in the drug in the past but contended he was not involved in the particular sale with which he was charged.

"You're a very lucky fellow," U.S. District Court Judge John H. Pratt told Jacomo after the verdict was returned yesterday.

Jacomo did not testify in his own defense. Watkins argued that Jacomo had been an innocent bystander in the cocaine deal and that it had actually been carried out by Jacomo's roommate who was then a waiter at the Palm.

The roommate, Alfred Latorella, pleaded guilty at the start of the trial to taking part in the sale to the undercover agent. As part of its plea bargain with Latorella, however, the government agreed not to call him as a witness against Jacomo.

"There was a sale, but it wasn't my client who did it," said Watkins in his closing argument. "It was Freddie Latorella."

Watkins portrayed the government's case against Jacomo as one built on informants and "plea bargains" with others who had been themselves deeply involved in drug activity.

Some attorneys acquainted with the Jacomo case described the government's evidence as unusually strong despite the use of such tactics, in that prosecutors presented evidence of the cocaine transaction from the time the order was placed with an out-of-town cocaine supplier until the time the money changed hands.

However, there was never any evidence that Jacomo himself had touched the drugs or directly participated in the narcotics being handed to the undercover agent. The government did present as evidence the tape of a telephone call between Jacomo and a drug informant in which Jacomo agreed to accept part of the money being offered in payment for the drug transaction.

The unusually youthful jury of seven women and five men, most of whom appeared to be in their 30s, deliberated for about one hour before returning the not-guilty verdict. The foreman, a man in his 20s, said later the verdict was based solely on the evidence and came because the government had not proved its case "beyond a reasonable doubt."

The government's key witnesses were a drug-dealer-turned-informant named Howard Engle Jr., who was born in his informant days as Jim Bone, and DEA undercover agent William Logay, who was introduced by Engle to others as Bill Lorenzo.

Engle said he had a previous drug transaction with Jacomo and produced a $2,700 check from Jacomo to document it, a transaction that Jacomo's attorney did not dispute.

The transaction with which Jacomo was charged occurred after Engle introduced Logay to Jacomo at the Palm and discussed the possibility of buying some cocaine early last February, according to trial testimony.

Engle said Jacomo told him that a shipment of cocaine would be arriving on Feb. 21 or 22, and to meet with him then. After other discussions over the next few days, Logay and Engle went to the Palm at 1225 19th St. NW on Feb. 22 and were given a sample of the drug by a waiter, they said.

Enfle and Logay said they went into a restroom at the restaurant to pretend to "snort" the coke sample, which they said was hidden behind the matches in a Palm matchbook the waiter had given him. Cocaine is normally ingested by sniffing or "snorting" it through the nostrils.

Later that night, the agent and informant said, Latorella actually sold them one ounce of the drug in the Palm business officer after Jacomo had motioned to the two men to go into the office.

According to their testimony, Engle and Logay paid only $1,000 of the agreed-upon $1,650 price that night. Logay said he thanked Jacomo for "letting me slide" on the remaining money until the next day.

The next day, according to a tape recording of the call, Engle asked Jacomo if he could deliver the remaining portion of the money to him at the restaurant and Jacomo agreed.

The only defense witness was former Palm waiter Dennis Bailey, who said the restaurant had a policy of having all packages for waiters delivered to the maitre d' first. Defense attorney Watkins then argued that Jacomo, as maitre d', was only stating that policy in the phone call in agreement to pick up the money for Latorella.

Leibowitz said Watkins' argument was "ludicrous" and described Latorella as a flunky for Jacomo in the drug sale. "Tommy Jacomo is a smart, intelligent man. He insulated himself" from getting personally involved in the handling of narcotics, Leibowitz said.

There have been at least three guilty pleas resulting from the cocaine investigation, that surfaced last March with the arrest of 20 persons. The investigation resulted in three different indictments alleging large-scale importation of the drug into the District and its alleged distribution in bars in the 20th and M Streets NW area.

The drug rings reportedly imported up to three pounds of cocaine at a time. Two trials involving numerous persons who participated in the alleged importation and distribution are scheduled later this summer before Judge Pratt.