On the few occasions that he talked about himself in his quiet, measured voice, Larry Wu-Tai Chin told acquaintances about his good fortune at the gaming tables of Las Vegas and his mathematical "system" of playing blackjack that had enabled him to supplement his government salary, buy real estate and help pay for his children's education, according to several of the acquaintances.

Chin drove a secondhand blue sedan, lived modestly at a succession of Alexandria apartment houses, and had only one apparent hobby -- his penchant for travel, according to those who knew the 63-year-old retired linguist and translator.

But the FBI has detailed a shockingly different version of Chin's life and source of his outside income, charging that for more than 30 years he was a spy.

According to the FBI, Chen sold U.S. government secrets to China and received more than $140,000 from Chinese intelligence agents after secret meetings in Hong Kong, Peking and Toronto.

The genial man who greeted neighbors warmly at the apartment building elevator and who occasionally sang Chinese opera with friends was stealing sensitive documents from his job at the Central Intelligence Agency, photographing them and delivering the film to Chinese agents in Toronto, according to the FBI.

He would then fly to Hong Kong for his payoff, receiving American currency that he deposited there, the FBI said.

Chin was employed until 1981 by the CIA's Foreign Broadcast Information Service in Rosslyn, where he was a GS-13 intelligence officer with access to classified materials, according to the FBI.

A former broadcast service employe who asked not to be identified said that most of Chin's work involved "overt" activities such as translation and analysis of foreign news broadcasts, but that he was also a member of "the production group" at the broadcast service and would translate "very sensitive documents," including those related to covert CIA operations in the Far East.

Although Chin was only a midlevel analyst, the potential ramifications of his alleged spying are significant, according to two sources familiar with the 23-month FBI investigation of his activities.

"The Chin case is much more serious than it looks," said one source, who declined to elaborate.

According to interviews yesterday with six of his associates, most of whom asked not to be identified, Chin was a quiet, highly intelligent man who was concerned about money, troubled by domestic problems and usually private about his family and his work.

One friend recalled, however, that Chin "would say that he worked for the CIA. I thought that was unusual, because CIA employes don't usually say 'I work for the CIA.' "

"He didn't talk much. The only personal thing he ever told me . . . was about Las Vegas," said Iris Denney, who rented a room to Chin during a period when he was estranged from his wife, Cathy.

Chin's wife, described as a former government employe who worked in broadcasting, lived on the 17th floor of the Watergate at Landmark condominiums, while Chin took a room on the 11th floor.

He "kept odd hours . . . came in late at night . . . typed and watched TV," Denney said.

Chin told several friends that he gambled heavily enough so that he was offered free air fare by Las Vegas casinos and made trips there as often as once a week.

It could not be determined yesterday whether Chin actually made the trips, but he discussed them with several friends.

Early this year, however, Chin reportedly told Denney that he stopped going to Las Vegas because casino owners barred "system" gamblers like him.

"He said they recognized him and he had this system, and that was how he acquired all his property . . . . I thought it was kind of out of character" because he seemed like a mild and careful man, not a gambling type, she said.

Chin told another friend he owned several Baltimore row houses and other real estate, and that he had used the proceeds of his gambling success to put two of his children through medical school.

Chin, who was previously stationed with the broadcast service in Okinawa and in Santa Rosa, Calif., has a daughter and two sons living on the West Coast.

Born in China and educated in Peking, where he attended a university, Chin was hired as an interpreter for the U.S. Army in Shanghai in 1948, but he was persuaded by Chinese communists to provide them with intelligence, the FBI said.

His spying began in 1952 when he provided information on the whereabouts of Chinese prisoners during the Korean War, and it continued after he retired in 1981, the FBI said.

Since retiring, Chin reportedly told friends he worked as a translator for the World Bank and other organizations. The FBI said he met with Chinese agents as recently as February.

"I have known him since the late 1960s, but I never really got to know him," said a longtime family friend. "We never talked politics. He didn't talk about himself.

"It was usually just small talk, like what happened in the shopping center, or what restaurant serves good food, but never about himself . . . . You think you know someone, but there is a certain part of their life you never touch."