Step by step, foot by foot, came the slow, steady march of Washington's diplomats at the State Department last night. More than 150 strong, in pairs, with wives, husbands, girlfriends, through the doorway of the John Quincy Adams room, onto the Oriental carpet, past the small desk upon which the Treaty of Paris was signed, step by step, foot by foot.

This was the reception where all of the ambassadors, and a fortunate charge d'affaires or two, came forward to shake hands with Secretary of State Alexander Haig (in brown suit, brown tie), his wife, Patricia (in a ming green dress), and chief of protocol Leonore Annenberg (violent patterned silk, large diamond and purple stone earrings, walnut-sized ring, clear plastic pumps).

"Hello boss," said Soviet Ambassador Anatoliy Dobrynin to Annenberg.

"She's my boss," Dobrynin explained to Haig.

The Israeli ambassador wasn't there, but the South African ambassador was, as were the ambassadors of Egypt, Algeria, Syria and the new ambassador from El Salvador. So were national security adviser Richard Allen and Seretary of Health and Human Services Richard Schweiker.

Down to Haig they all came, their name cards passed along through no less than four State Department staffers, each of whom read the name and whispered it on down the line to Annenberg, who then made the announcement loud enough for the Haigs to hear.

Of course, finding an original greeting for more than 150 diplomats is not particularly easy, but the secretary of state did his best.

"Why, sure!" said Haig, shaking the Guatemalan ambassador's hand.

"Yes, nice to see you," he said to the council general of Estonia.

"Well!" he exclaimed. That was to the ambassador of South Africa.

"We're with you, you know that," Haig told the ambassador from El Salvador to the OAS.

"We're all working for the same cause," Haig said, grasping the hand of the ambassador of Lebanon.

And then there was the ambassador of Japan."How are you doing?" Haig wanted to know. "Golf, a little golf?"

Mrs. Haig shook hands graciously and allowed her cheek to be kissed from time to time, often turning her head to give a departing diplomat's wife the once-over from behind.

So it went. And after a time, step by step, foot by foot, the diplomatic corps passed by, into another, larger room for drinks and hors d'oeures. Just what kind of hors d'oeuvres will remain forever a secret.

"I'm sorry," said a State Department staffer who was guarding the press, "we can't give that information out."