IN WHAT WAS once the ornate and formal conference room of Ambassador Ardeshir Zahedi's Iranian embassy in Washington, three stocky, bearded students sit with their feet on the mahogany table. They are members of the 40-man "interim committee" whose task is to purge the chaff from the wheat among the 55,000 Iranian citizens in the United States.

Across from them, chain-smoking, sits a borderline case: a white-haired career diplomat who has expressed intelligent, unemotional criticism of the former Iranian government since it fell, along with a cautious willingness to work for the new regime. The three Khomeinist committee members are grilling the diplomat about his role as an emissary of the shah, and trying without success to draw out his views on the reactionary or revolutionary qualities of his former colleagues.

To some degree, anyone's willingness to work for Premier Mehdi Bazargan in Washington is academic. Although Djafar Faghigh, formerly No. 3 under Zahedi, has just been replaced as Bazargan's charge by a career diplomat from Tehran, Hassan Etessam, the embassy is really run by Shahriar Rouhani, the son-in-law of Vice Premier Mohammed Yazdi. And neither Rouhani nor the members of his committee will take orders from Bazargan, Yazdi or anyone else in Tehran.

They have pulled the plug on the telex which connects them to the foreign ministry. Unique among Iranian embassies abroad, the mission in the Persian palace building on Massachusetts Avenue takes its orders solely from the Ayatollah Khomeini in Qum.

THE GRILLING goes on all afternoon. It is worse, the shaken victim says later, than the loyalty tests he has had from Savak, the shah's secret police. Why, he is asked, did he not speak up sooner against the shah?

He looks at the gilt ceiling and begins counting on one hand. He recalls only three Iranian career diplomats who, over the past 20 years, had voiced some mild criticism of the shah's policies. All had ended up with dead-end jobs, usually in Tehran.

The committee begins asking about Zahedi's alleged $50,000-a-month slush fund, used mostly to buy gifts-from caviar to jewelry-for the influential. Did the diplomat ever distribute funds for Zahedi on Capitol Hill?

"My dear fellow," says the diplomat politely to the questioner, carefully fingering ash into a blue-and-white mosaic tray, "that is not how Zahedi worked. If he had worked to approach a senator, he would have called him to the embassy for lunch or dinner. He would never have gone to the senator, and even less entrusted the mission to someone else. He didn't conduct diplomacy that way."

Didn't they know, the diplomat wonders. Literally everyone would gladly accept a Zahedi invitation. His receptions glittered. Champagne, caviar and prepaid call girls were plentiful. His subordinates took their cue from the master. If a reporter asked an Iranian diplomat to lunch in a restaurant, the diplomat would always insist on paying, always with a $100 bill and a "Keep the change!" to the waiter.

Now only five of the 36 diplomats remain at their posts. Zahedi is in Montreux, Switzerland. A few of his staff have returned to Tehran to take their chance on survival, but most are on "extended leave" in America.

The five who take their orders from Rouhani are laughed off by the exiles as opportunists. They are the ones who first took a stand against Zahedi (but not the shah, who was still in power) and this has been their passport to partial acceptance by the committee, which has no idea how to run the day-to-day business of the embassy and needs their help.

Those whose loyalty to the revolution is being questioned are the ones who signed a statement supporting the "struggle of the Iranian people to attain genuine democracy," but left out any mention of the shah or Zahedi.

AFTER THREE hours, the white-haired diplomat is allowed to go home-with a long questionnaire to which he is supposed to respond in detail. He is shaken. Could a career civil servant have behaved any differently over the years? Was no one giving credit for what Zahedi had done?

He had projected Iran on the Washington scene, and Washingtonians had seemed to approve. Did the progressive young men of the committee know how ardently Zahedi had defended the Palestinian cause on Washington's power circuit, how he had always refused to talk to the Israeli ambassador, how he had convinced the shah to allow Amnesty International to investigate torture in Iran? Did they know the skill with which he had cultivated relations with each succeeding U.S. administration? Did they care?

It was true, he reflected, that Zahedi had had his faults. Although he had constantly warned the shah that the CIA and U.S. forces would never come to his aid again, he had underestimated how widespread congressional criticism was. He had always been more of a wishful thinker than an analyst, and he had failed to pierce the sycophancy of his paid adviser, former Secretary of State William P. Rogers.

But then, it had always been hard to tell the shah the truth, if the truth was hard. Right to the end, the shah had said that his destiny would be determined by Washington. He was right, of course, but not in the way he would have wished.

And, after all, even Zahedi was luckier than some. Had he been given the post he coveted - minister of the court - he would probably be stretched out in the Tehran morgue by now.

The diplomat returns home. His heart is still racing from the interrogation. He pours himself an un-Islamic scotch and soda. The telephone rings. It is one of the other exiles. Has he heard the news? The committee has ordered Faghih himself to face interrogation! And Faghin has protested to Tehran! He should have protested to Qum, says the diplomat. The two men laugh. It is good to be relieved of tension for a while.