At the center of a terrorism prosecution now under way in federal court is a promise allegedly made during a meeting of three men in a Bronx apartment on a Friday morning two years ago.

The eldest man present was a medical doctor educated at Columbia University who had a hospital job in Saudi Arabia. The youngest was an accomplished agent at the Federal Bureau of Investigation who was posing as a recruiter for Al Qaeda. The two had never before met. For nearly three hours, they sat on the floor sipping mint tea. Around noon, the undercover agent, Ali Soufan, allegedly told the doctor, Rafiq Sabir, to repeat after him, and Mr. Soufan administered the oath of allegiance to Al Qaeda.

Prosecutors played a tape recording of the conversation yesterday in United States District Court in Manhattan, where Dr. Sabir is being tried on charges that he conspired to use his medical expertise to treat wounded Al Qaeda operatives.

On the tape, Mr. Soufan suggests the possibility that, sometime in the future, Dr. Sabir, 52, would be in Afghanistan, where he could voice his oath to Osama bin Laden in person. In the meantime, Mr. Soufan says, he can act in Mr. bin Laden’s stead and take his pledge.

“I have an oath and a promise to God, to obey the guardians of the pledge, to exalt the word of God, and to be protective of my brothers on the path of jihad,” Mr. Sabir repeats in Arabic after Mr. Soufan on the tape. “And to protect the secrecy of this oath, and on the path of Al Qaeda.” Dr. Sabir stumbles over the word ‘Qaeda,’ prompting a correction from Mr. Soufan.

On the tape, Dr. Sabir offers little insight into why he took the oath, except to say that he and the third person at the meeting, Tarik Shah, “have talked about this for a long time.”

Shah, who introduced Dr. Sabir to Mr. Soufan, made the same pledge that day. He has since pleaded guilty to conspiring to teach martial arts to Al Qaeda operatives.

Shah dominated the conversation that day, judging from the tape recording, speaking on matters ranging from his son’s troubles with the law to his plans to build a martial arts studio in his courtyard. Dr. Sabir speaks the most when Shah, whose apartment they were using, left the room to place rat poison upstairs, according to the tape.

Alone with Mr. Soufan, Dr. Sabir boasts that, as a doctor on a Saudi military base, he had a card that allowed him to move freely about Saudi Arabia.

When the FBI agent tells Dr. Sabir that he will be a valuable asset for Al Qaeda in Saudi Arabia, where he will be able to help mujahideen who are hurt, Dr. Sabir agrees, according to the tape. But the plans are not specified any further before Dr. Sabir changes the topic to share his suspicion that his name is on a no-fly list, according to the tape.

Mr. Soufan, who now works at Giuliani Security and Safety, testified that the oath he administered is one that is used by Al Qaeda. Mr. Soufan, who investigated the bombing of the USS Cole and the terrorist attacks of September 11, 2001, said he learned the wording “through my interviews with Qaeda guys who cooperated.”