IF JAMES W. ROUSE didn't happen to be one of the country's most progressive, imaginative and successful developers, we might have more than a few doubts about news that a private firm -- the first of its kind -- is being organized to help turn slums into good, low-income housing. But Mr. Rouse has a solid record of social concern and -- just as important, when you're talking private business -- of real results; and it is he who is behind a resourceful new venture aimed at stepping in where federal housing money isn't so available any more.

Mr. Rouse, whose credits include Harborplace in Baltimore, Faneuil Hall Marketplace in Boston and The Gallery at Market East in Philadelphia, says the new company, Enterprise Development Corp., will be owned by a non-profit organization, the Enterprise Foundation, that will distribute the company's money for housing improvements. Though this arrangement was conceived before President Reagan took office, Mr. Rouse points out that the new administration "certainly brings on the crucial necessity for inventing new processes, new systems for dealing with social needs."

All too often, however, such talk in developer circles has meant turning slums into luxury housing -- and never mind where the poor wind up. What Mr. Rouse has in mind is different; the type of support the new firm will give will be patterned after the impressive Jubilee Housing Corp., which has worked to improve apartment-building conditions in the Adams-Morgan neighborhood.

The company will work with communities' residents in making improvements and will use small local developers -- providing them with technical assistance and equity help. In exchange, the commpany is to receive 70 percent to 90 percent ownership in each project. Eventually, proceeds from these ventures will support other housing projects.

Obviously, private enterprise is neither equipped nor inclined to substitute entirely for federal assistance in improving and/or increasing the supply of housing in this country. But if the initiative of Mr. Rouse does demonstrate that non-goverment efforts can be attractive propositions, it will be a most important contribution to "supply-side housing" for the very poor.