They were singing the First Lady's praises yesterday along New York's Seventh Avenue, the same place where only two months before they had been voicing some doubts about her 6-year-old Inaugural Ball gown, in particular, and her fashion taste, in general.

Rosalynn Carter, you see, had just bought her spring and summer wardrobe.

It was like music to the ears of the hard-pressed International Ladies money in the bank to at least four New York design houses.

In six hours on Wednesday, the First Lady had bought a couple of coats, again that many suits, several daytime dresses and at least two gowns for evening. And if she caught some designers by surprise - "The White House called just hours before she arrived," said a spokesman for designer Jerry Silverman - the ILGWU had been prepared.

In fact, the ILGWU's president, Sol Chaikin, during what a spokesman called "just a friendly visit" to President Carter on March 15, set the trip in motion when he expressed the hope that Rosalynn Carter might like to visit Seventh Avenue sometime.

"That's where it all began," according to Evelyn DuBrow, the ILGWU's legislative director, who, with executive vice president Wilbur Daniels and assistant president Gus Tyler, was waiting for the First Lady Wednesday when she arrived at Silverman's showrooms.

Chaikin, himself, couldn't join them since he was hospitalized for minor surgery but his wife represented him, accompanying Rosalyn Carter, her sister-in-law Ruth Carter Stapleton and the First Lady's personal assistant, Madeline MacBean, on their swift fashion rounds.

Badly hit by what DuBrow called "the penetration of imports - 89 per cent of the garments sold in the United States today are imports," the ILGWU has been losing members steadily as domestic production has dropped.

"We wanted to indicate," DuBrow said of Rosalynn Carter's Wednesday visit, "that we were encouraging Americans everywhere to buy union-made clothing."

Said a spokesman for Sol Chaikin yesterday in New York: "It (the Carter visit) was a tremendous boost for the industry."

It was also a tremendous boost (if not necessarily in money, certainly in publicity) for some individual designers. At Silverman where the First Lady made selections "right off the rack," according to sales manager Eli Lehman, she ordered several articles in the imitation suede fabric, Ultra-suede, (three-piece suit, coat and jumper), plus a pencil-striped suit with crepe de chine shirt.

At Dominic Rompoilo, who designed her Inaugural wardrobe and also added a wrap to update her ball gown, she selected six items. Among them were an all-white matte jersey floor-length dress and coat, a long "Rosalynn green" matte jersey gown, a linen jumper in banana yellow, a cotton apricot and white-striped daytime dress and another in "peanut" and white.

At Abe Schrader, she chose a navy-blue coat, aqua dress and four-piece navy linen suit with vest.

She also looked at sketches at Kasper.

"She knows pretty well what she wants and what she likes," said Rompollo.

From Seventh Avenue came indications of a welcome truce. Said Halston, who only two months ago had accused the Carters of accomplishing "a real putdown" of the fashion industry by their low-key treatment: "I think it was very gracious and very nice of Mrs. Carter to go to Seventh Avenue and be supportive of our industry."

Equally positive was 1976 Coty Award-winning Mary McFadden, who called it "important" that the First Lady had made the visit.