It must have seemed an unusual sight out there at Amway in Ada, Mich., on Dec. 2. What in the world was Mstislav Rostropovich doing there entertaining employes of the grass-roots direct sales giant during their lunch break? Was it to enhance the billion-dollar sales of the international grass-roots purveyor of detergents, panty hose, vitamins and you name it for the home? Well, in a way, yes.

That's because Amway, out to nurture its European image as markets expand there, is bankrolling the month-long tour of Europe by Rostropovich and the National Symphony, which begins Monday. "Slava has high visibility there," declared Jay Van Andel, Amway's board chairman and co-founder, in a recent interview. "He communicates better than most music directors do... It fits in with Amway's advocacy program."

Speaking for a politically conservative company that merchandises free enterprise ideology as aggressively as it does soap, Van Andel observed, "We heard about Rostropovich and we liked the kind of philosophy he [the emigre Russian] has about freedom."

Rostropovich likes Amway, too, because Amway came up with the $250,000 -- to finance the tour that Rostropovich regards as the most important test yet of his five seasons with the National Symphony -- just as it appeared last summer that tight money would force its cancellation.

As a favor he flew to the suburbs of Grand Rapids for his luncheon musicales with the Amway brown-baggers after his fall series of concerts at the Kennedy Center had concluded. Casey Wondergem, Amway's public relations chief, described the scene:

"We would bring in about 350 at a time at our company auditorium. We brought in about two-thirds of the people from the factory, and the others from the offices. We provided the brown-bag lunches. Slava would stand up and tell stories for about the first 15 minutes, mostly about his cello [the Duport Strad of 1711].

"Then he played four short pieces, including one by Shostakovich, and before it he talked about his close relationship with the composer. He was a great hit, even with a lot of people who before that didn't even know what a cello was."

If the marriage of Amway and the orchestra seems an unlikely one, the way they came together is even stranger. Amway found out about the National Symphony tour from a catalogue. Produced by the Netherlands-American Bicentennial Commission, of which Amway's Van Andel is chairman, it offered 40 projects to which corporations might contribute to observe this year's Dutch bicentennial. The NSO's proposed concert in Amsterdam was advertised for $40,000. "It was almost a fluke," said Van Andel. "Nobody came to us. It was just right there in the catalogue. It was on page 37."

What would a huge Michigan corporation that was started from scratch only 21 years ago and is directed primarily toward the suburbs and the middle class have to gain by sending a highbrow orchestra based hundreds of miles from Michigan to Europe for a month?

"Well, of course, in the sponsorship of cultural activities there has to be a return to the business," Van Andel said. "This is the biggest cultural involvement from the standpoint of a single event that we have ever made. Here the return is in terms of the attitudes of the general public. Our European market is new, but it is growing very fast. We now have about 100,000 distributors there. We can bring in our top distributors to the concerts and to the parties that will follow them. Distributor pride in Europe is very important." Van Andel will join part of the tour, but his partner, Amway's president and co-founder Richard DeVos, will not.

And in its way the National Symphony is going to Europe for image purposes, too. Though Rostropovich is quite famous there, the National Symphony is not well known. In recent years, the orchestra has not undertaken a large-scale European tour (one was scheduled under former music director Antal Dorati but had to be canceled for financial reasons).

To justify the expense of such a trip, there must be a reasonable expectation that there will be a return to the orchestra in terms of its reputation. Bad notices from European critics and indifference from European audiences can be more damaging than nothing at all.

Rostropovich has carefully planned the tour to show the National Symphony at its best. There will be no soloists to share the spotlight (they performed with soloists on their tour of the Far East two years ago).

In his programming, Rostropovich has deliberately avoided the experimental or unfamiliar. "I tell you why," he said in an interview last month. "I don't want to play anything that somebody might not like, so that they end up talking and writing about the composition and not the performance. It cannot overshadow the performance. It's our first visit together. I don't want to take any risk for our orchestra."

Does he expect success? "I hope... I hope." Then he added, "It's not that I am afraid of this tour.But it's a pleasant anticipatory excitement."