A CHICAGO RETAILER has an idea that Washington businesses might well adopt. The retailer, Sol Polk of Polk Bros. appliance stores, is offering Chicagoans free mass-transit rides to his stores. Under the system, which he calls a FARE Plan, the stores refund the equivalent of a round-trip transit fare to everyone who brings in a transfer no more than a few hours old. Riders don't have to buy a thing to get the refund. "As long as they came in and see what we've got, we're happy," says Mr. Polk.

Mr. Polk obviously wants to sell more appliances - but he's trying to sell mass transit, too.He thinks the nation can't afford to burn more and more gasoline in private cars and let its vast investment in mass transit go to waste. Buses and subways should be marketed like any other product, he believes. So his company has invested $200,000 in rebates and promotion for a 90-day test of the FARE Plan. The results so far are good. Despite very bad weather last month, Mr. Polk reports, his stores' sales surpassed last January's by 12 percent. The plan has brought in "a lot of nice people," he says, including many "who haven't been on public transportation for years."

It's a splendid idea. Retailers routinely subsidize people who park and shop; why not encourage folks to ride and shop instead? Fare refunds should also draw potential customers such as elderly people who don't drive. And everbody benefits when public transit is used more, especially in off-peak hours. Washington retailers should try something like this; the ideal time might be when Metro starts running on Saturdays. It strikes us as the kind of private initiative that can produce profits for everyone.