At least one -- and possibley many more -- of the American hostages who have been spread around Iran is living under guard at 70 Kamal Esmail Avenue here in Isfahan, the architectural showpiece city of a 16th century Persian monarch.

The house, formerly owned by an informer for SAVAK, the dreaded security police under the deposed shah, is situated in one of the best parts of this central Iranian city. It is well guarded, featuring a sandbagged sentinel post atop the entrance gate, numerous self-appointed vigilantes and the omnipresent anti-American banners of Iran's Islamic revolution.

The price for discovering yesterday that the house, mostly hidden by an imposing 60-foot-long wall, is a hostage site was considerable. For this reporter and Doyle McManus of the Los Angeles Times, it cost 4 1/2 hours of detention by Revolutionary Guards, a search of our hotel rooms and being ordered to leave the city by 8 a.m. today.

The site is not hard to spot since it has a prominent sign saying it is the headquarters of "Students Following the Line of the Imam," Iranian leader Ayatollah Ruhollah Khomeini. The U.S. Embassy in Tehran, which the militant students took over last Nov. 4, has similar signs.

Several sources have said in recent weeks that they thought some of the hostages had been moved to the Isfahan house after the militants at the embassy decided to split up their captives following the abortive American resue attempt in late April.

The militants have said they have scattered the hostages around the country, leaving considerable questions about how many of the 49 remaining captives from the embassy are in the capital. Three other American officials are at the Irainian Foreign Ministry.

The reception outside the gate yesterday for two reporters and a translator was unfriendly, as a Revolutionary Guard in the sandbagged position pointed his rifle at us. But it did not take long to gain confirmation that there was a hostage inside.

Three students came out to talk -- or more accurately to discourage talk.

"We will not tell you anything." said a tall student in a blue shirt. Then, when asked whether the estimate of six hostages at the site was accurate, he said, "there are between one and 50."

Soon we got further, unwelcome confirmation.

It was impossible to get a closer look at the embassy. The students took offense at our copying down revolutionary slogans from the wall, and ordered us to the nearby Revolutionary Guards headquarters, where our notes were temporarily seized.

While he was intensively questioning me for 2 1/2 hours, one of the security agents at the Guards' office said repeatedly that there were hostages down the street at 70 Kamal Esmail Avenue.

The Revolutionary Guards office had formerly been the local headquarters of SAVAK, and the surrounding buildigs for about a quarter of a mile had been given to SAVAK's informants to reward their services, the interrogator said.

It seemed highly unlikely to me that the security agent was seeking to mislead me about the hostages by planting false information. The desultory questioning was apparently aimed at trapping us in inconsistencies in our separate interviews.

One telling point was frequent reference to alleged involvement of Western reporters with the Central Intelligence Agency. The interrogator cited acknowledgement of such employment by CIA officials and asked, "how can you expect us to believe" that American journalists "do not work for the CIA?"

The interrogation was carried out by five security agents, one openly holding a pistol in a holster on his side. Unlike a SAVAK interrogation experienced under the shah's regime, however, there was no feeling of potential violence, only uncertainty of release and inability to contact anyone outside the headquarters.

The impression was of an independent authority acting on its own beyond control of the central government.

Several other foreign correspondents have been held longer or expelled, and many have been refused entry since the revolution, which used to be a favorite response of the shah's government to unfriendly reporting.