In the early 1970s, members of a Japanese acting troupe noticed a small, elderly man guarding the door of the Peking People's Art Theater, where they had come to perform.

They thought they recognized the man, and they were right, Cao Yu, the theater's former president and perhaps China's foremost 20th-century playwright, was in the midst of a 10-year disgrace orchestrated by the Gang of Four. Making him a doorman, the gang had decided, would be just the punishment to fit his crimes.

And what were his crimes? The worst of them, Cao now feels, was writing a play that Liu Shao-chi, a Chinese leader who had been discredited by the Gang of Four, happened to praise. The play was "Thunderstorm," a realistic chronicle of a rich family in Nationalist China, which Cao had originally written in 1934.

For years after Cao's detention in 1966, interrogators pressed him about his alleged ties to Liu. But Cao could remember no ties except Liu's three sentences after a revival of "Thunderstorm": "Profound," the former head of state had said, "Very profound. Extremely profound.

"I disappointed them because I really couldn't think of any crimes," Cao, now 70, recalled yesterday during a hasty interview at the Kennedy Center, where he delivered a lecture on current Chinese theater and lunched with a group that included Center chairman Roger L. Stevens.

During the interrogations, Cao was questioned about relations with Chiang Kai-shek. His interrogators, he explained yesterday, had decided that a historical parable play ("The Gall Bladder and the Sword") he had written in opposition to the Soviet Union's conduct toward China was really an attack on the revolution and a call for Chiang's return to the mainland.

After the Japanese recognized him at the theater, there were headlines in Tokyo proclaiming, "China Shakespeare Now a Doorkeeper." The Gang of Four decided his status was "a little embarrassing," Cao said yesterday through an interpreter. So they moved him to a less conspicuous door, guarding a dormitory.

For 10 years in all Cao and his theater colleagues were under collective house arrest while Red Guards attempted to re-educate them. Writing was "impossible." For nearly five years, Cao was sent into the countryside to "feed pigs and grow rice . . . under the surveillance of the masses."

Born in 1910, Cao was the son of a rich family of scholars and government officials in the port city of Tianjing. At 5 or 6, as he told yesterday's audience, he discovered a classical Chinese translation of "Lamb's Tales From Shakespeare." Later he read the novels of Dickens and the plays of Ibsen, who became the greatest Western influence on Cao as a playwright.

While still a student, he wrote "Thunderstorm," an epic play of a coal-mine-owning family and the relationship between servants and masters. "Thunderstorm" became a rallying point for students and intellectuals agitating against the corruption and feudal trappings of Chinese society in the '30s. (Two subsequent plays, "Sunrise" and Peking Man" were performed in New York last week in honor of Cao's visit there.)

The Gang of Four "wrought such havoc on the country," said Cao. Every corner was contaminated . . . They ruined a whole generation of children." Their overthrow was a "second liberation."

The last few years have been "better than ever," he said (and the presence of Chinese ambassador Chai Zemin at yesterday's lecture seemed to affirm Cao's restored standing). There are no restrictions on his writing under the current administration, according to Cao. "We have come to a day when the leaders and the masses want you to speak the truth. It's no longer taboo to point out the defects, the drawbacks of our present society."

"The best of the new novels and plays," said Cao, "not only have the courage to expose what is unpleasant but also do it with an optimistic spirit." c

Cao described two current Chinese plays in his Kennedy Center address -- "Authority and the Law," the story of a corrupt party official who, in Gang-of-Four times, misappropriates money meant for famine relief; and "Save Her," the tale of a worker's daughter who, after trying to assume a false identity to get into a university "through the back door," is disowned by her parents and friends when the falsehood is exposed.

Such plays illustrate a new willingness to address painful problems and to create "flesh and blood" heroes, Cao said. But he interrupted his lecture later on to point out that the acts of corruption depicted in these plays were "isolated cases." "One musn't feel that every official is that way," he said.

Cao has been to see "After the Fall" at Arena Stage and "The Elephant Man" at the Kennedy Center. No American plays of the 1970s have been performed in China, he said. In fact, "we've been cut off for so long that what has happened for the last 30 years in the West we know very little about. But the willingness now to read western plays is very great."

The People's Theater plans to produce "Macbeth," "The Merchant of Venice" and Moliere's "The Miser" this year. Next year, said Cao, the theater hopes to do one of Arthur Miller's plays. Can has had a number of meetings with Miller, both in China and in the United States, where the two writers have "talked shop."

As for his own future, Cao hopes to make up for "so much time lost" by writing "one or two more plays that will be better than my other work."

A friend recently quoted an old Chinese saw that Cao appreciates: "The scholar is now aged and he values his remaining years."