The concept of a Baltimore-Washington common market appears to be gaining wider acceptance in all but a few hard-core pockets of parochialism.

Business leaders in Baltimore and Washington have formed a partnership to promote the region as one economic entity, despite considerable early resistance to the idea among many of their colleagues. In fact, the nonprofit alliance called the Washington Baltimore Regional Association sees its role as that of a catalyst for economic growth in the region anchored by the two cities.

To be sure, city and county officials in the Baltimore-Washington region have their own economic development programs that focus on narrower interests. That is how it should be.

The two metropolitan areas complement each other, nonetheless, and their combined resources make the 40-mile corridor considerably more attractive to investors than either one individually.

Economic analyses prepared for the WBRA show that the Baltimore-Washington common market is the fourth-largest in the nation, the most populous and the leader in median household income.

Local retailers, real estate developers and financial institutions, among others, concluded long ago that their markets extend from Northern Virginia to the northern fringes of the Baltimore Beltway. And, led by the big Baltimore banks, businesses from that city have become as much an integral part of commerce in metropolitan Washington as locally based firms.

This blurring of traditional economic and geographic boundaries has been dictated largely by consumer demands, household growth in the region and competitive forces leading to expansion in the marketplace. The results of those phenomena were reflected in part in The Washington Post's annual census of the area's top 100 companies earlier this month.

Surprisingly, The Baltimore Evening Sun missed the point entirely and took up the banner of parochialism in a May 5 column. The Evening Sun is concerned, we learned, that Washington has annexed Baltimore and that it has changed the addresses of some venerable Harbor City companies.

Where could the Evening Sun have gotten such a preposterous idea? Alas, from The Post, we're told. The Post "has publicly given notice that the nation's capital has annexed Baltimore completely," The Evening Sun declared.

Absurd, you say? Well, The Evening Sun--sans tongue in cheek--bemoans the fact that the Post's list of top 100 companies includes firms such as Baltimore Gas & Electric Co., Loyola Federal Savings and Loan Association, Maryland National Bank and McCormick & Co.

"The last time I looked, all those firms were in and around Baltimore," The Evening Sun's columnist declared.

The last time I looked, BG&E was providing electric and gas service to 50,543 customers in Montgomery and Prince George's counties just outside Washington.

The last time I looked, Loyola Federal had seven offices in Washington's suburbs. Maryland National Bank, the region's largest, has 52 branches in Montgomery and Prince George's counties. What's more, Maryland National has loan production offices in the District as well as in Northern Virginia, where it also operates an Edge Act subsidiary.

The unmistakable aroma of spices around Baltimore's Inner Harbor pinpoints McCormick's address. But McCormick's real estate division is developing a major business park on the Beltway in Prince George's County, less than 10 miles from downtown Washington.

Obviously these and other Baltimore-based companies have come to accept a broader view of the geographic boundaries of the marketplace. And the common market, where they're concerned, is the Baltimore-Washington corridor.

As The Post's Jerry Knight pointed out in his May 2 Monday Morning column on the top 100, companies from Baltimore, Richmond and beyond clearly aren't Washington businesses. Knight added, however, that The Post 100+ is unique in its emphasis on companies that "Washington-area residents are most likely to invest in, work for and do business with."

That notwithstanding, The Post 100+ list not only identifies the top 10 Maryland and Virginia companies but includes a separate grouping of the 25 largest financial institutions serving the area.

To their credit, managers of those companies have abandoned provincial concerns in favor of new corporate strategies. In Maryland National's strategy, for example, Washington's Maryland suburbs represent a "contiguous natural market area acknowledged as the state's 'power alley' for commercial and retail economic growth."