The American hostage crisis hung over last night's Ambassador's Ball, the 37th annual event culminating the Israeli Bond Drive, as surely as did the brilliant red baskets of carnations. The speeches were serious, the guests thoughtful.

Actor Elliott Gould, the irrepressible star of the movie "M*A*S*H," was as serious as anybody. "I've been a Jew all my life," he said. "I grew up praying for the unification of Jerusalem."

Standing in the receiving line at the ball at the Washington Hilton, the actor looked very serious in black-tie -- until observers began to point to Gould's feet, ensconced in blue and white tennis shoes. "They're comfortable," Gould defended himself, admitting that at Size 13-D they weren't altogether unnoticeable.

"He must not know about Roc-Sports," said surgeon Fred Schwartz. "I wore mine." Currently, the thick-soled but traditionally styled black shoes are becoming as standard as black ties for men who go on to the party circuit after a day on the treadmill.

Very serious indeed was Israeli Ambassador Meir Rosenne, who said that, yes, he had been awakened in the middle of the night to learn of the hostage crisis. But, he said, that was part of the job of being a diplomat. "That is a small thing compared to what the hostages are going through. Terrorism is everywhere. It must end."

Later, in his speech, Rosenne said, "At this moment, American hostages suffer because the United States is the greatest democracy. We suffer because we are the smallest democracy. We are fighting for our life. Our hearts go out to the Marines killed in Lebanon, the Jewish athletes killed in Munich. But let no one make a mistake and think this crisis will make a rift between the United States and Israel. We stand together. We shall overcome."

For Israel, the evening had a happy side. "We are celebrating the sale of $6.5 million in Israeli bonds in the Washington area," said ball chairman Estelle Gelman, a Washington real estate executive. "The 900 people who came here tonight bought $5,000 in bonds per couple. And the dinner was paid for by our group of patrons."

Representing President Reagan at the ball, Education Secretary William Bennett said in his speech: "I bring the president's greetings. Israel and the United States stand for common principles -- justice and the rule of law. It would be wrong not to sound a somber note tonight. The hostages are not far from our mind. Israel, has, in similar situations met them with courage and humanity. We now find ourselves in such a situation. We must prevail."

U.S. Information Director Charles Z. Wick said he came "because the ambassador invited me. He's a gracious man."

Rosenne gave awards to Gould and to singer and actress Lainie Kazan for their friendship with Israel. In his acceptance speech, Gould said he had always considered himself an American first and a Jew second, until he went to Israel.

The reunification of Jerusalem 18 years ago was the theme of the evening, symbolized, Gelman said, by the word "chai. The word means both 18 and life."