After months of painstaking groundwork, the Italian judicial probe of the May 1981 shooting attack on Pope John Paul II has yielded its first major results, which directly implicate Bulgarian citizens and could have a significant impact on East-West relations.

Italian law enforcement agencies have taken the Bulgarian trail into consideration ever since it first became known that Mehmet Ali Agca, the young Turk captured in St. Peter's Square moments after he shot the pope, had spent several months in Bulgaria after his escape from a Turkish prison in November 1979.

An Italian police report dated May 27, 1981, published yesterday by the Italian weekly Espresso, named a Bulgarian known as "Mustaeof" as a possible suspect along with 17 Turks and a Syrian.

Bulgaria is one of the Kremlin's most loyal allies, and it is believed that the Soviets may have sought to kill the pope because of his support for the outlawed trade union Solidarity in his native Poland.

But until the Nov. 25 arrest of Bulgarian airline official Sergei Ivanov Antonov on charges of "active complicity" there was no concrete evidence that investigators believed Soviet Bloc agents were involved.

Following Antonov's arrest, Judge Ilario Martella, the chief Italian investigator, made a rare public pronouncement, saying that any conjecture about the existence of an "international plot" was "unfounded and premature."

But the theory appeared to win greater credibility here this weekend when judicial and police sources made it known that an arrest warrant has also been issued for Jelio Kolev Vassiliev, a former staff member of the Bulgarian Embassy in Rome, and that a current embassy staff member with diplomatic privileges is also under suspicion.

In recent months the Bulgarians have been on the minds of many Italians. Last spring one of the kidnapers of U.S. Gen. James L. Dozier said the Bulgarians had offered help to Italy's Red Brigades terrorist group. A left-wing Italian trade unionist is in jail on terrorism and spying charges that involve alleged contacts with the Bulgarians. And in the northern city of Trento, examining magistrate Carlo Palermo recently acknowledged that Bulgarians were implicated in a vast smuggling network trading arms for heroin that he is investigating.

One name mentioned in reports on the smuggling network is that of Abuzer Ugurlu, a major Turkish criminal who reportedly met Agca in Bulgaria and was named in U.S. media reports as a key link between Agca and the Bulgarians. Martella flew to Ankara to question Ugurlu who, the magistrate said in an interview earlier this fall, denied any involvement in the assassination attempt.

Despite the caution expressed by investigators and observers, the notion of a Bulgarian role has consistently won greater credence. Yesterday, for example, a special Italian appeals court denied a request for Antonov's release, ruling in effect that the evidence of the Bulgarian's complicity presented by Martella justified his unprecedented arrest.

Some of the latest developments in the case appear to stem from a new loquaciousness by Agca, who may be making good on a promise made at the time of his trial in July 1981 to "talk" if he were not helped to escape within six months.

According to judicial sources, Agca provided much of the evidence leading to Antonov's arrest, reportedly furnishing detailed descriptions of the Bulgarian's Rome apartment.

Unconfirmed press reports claim that Martella convinced Agca to talk by informing him that the original "getaway" plan from Saint Peter's Square had included his own death.

Five Turks and three Bulgarians now have been implicated. Besides Agca, they are:

* Sergei Ivanov Antonov, a Bulgarian who was head of the Balkan Airlines office in Rome for the past four years. Unconfirmed reports say he is suspected of helping Agca plan the final stages of the assassination attempt, including a getaway plan, and to have reserved a room for Agca at a boarding house near St. Peter's Square. Although a photograph published last week showed a man who bore a strong resemblance to Antonov close to the pope at the time of the shooting, investigators here are said to have identified the man as an American tourist.

* Jelio Kolev Vassiliev, former assistant to the Bulgarian Embassy's military attache, who returned to Bulgaria last August, according to an embassy spokesman and who has now been charged by Martella.

* Teodorov Ayvazov, an embassy cashier who is now in Bulgaria on vacation. Although not a diplomat, he has some diplomatic privileges and Italy reportedly has asked that these be removed so a warrant can be issued. An embassy spokesman said it was uncertain whether Ayvazov would return to Rome.

* Omer Bagci, a right-wing Turk arrested in Switzerland last June and extradited to Italy in October on charges that he met Agca in Milan a few days before the shooting and supplied him with a 9mm pistol.

* Musa Cedar Celebi, a right-wing Turk heading a Turkish cultural organization in West Germany. He was arrested in Frankfurt Nov. 3 on charges of complicity in the planning and execution of the assassination attempt. Martella flew to Frankfurt to interrogate Celebi and later issued other arrest warrants.

* Bekir Celenk, a Turkish smuggler, was charged with complicity in the crime by Martella on Nov. 27. Celenk, who according to a Turkish newspaper has taken refuge in Bulgaria, is said to have met Agca in Sofia in July 1980 and offered him 3 million German marks to kill the pope.

* Oral Celik, sought by Turkish police both for murder and helping Agca escape from prison. Celik was charged on Nov. 27 with complicity in the plot.