The Earle Palmer Brown Cos., Washington's largest advertising and public relations agency, has acquired Philadelphia's Spiro & Associates Inc. and will merge the firm with its own office there, making it the second-largest ad agency in that city.

The price EPB paid for the privately owned firm was not disclosed. The Spiro agency, when combined with Earle Palmer Brown/Philadelphia, form a company with billings of more than $87 million and about 175 employes, EPB officials said yesterday.

"It's a big step in terms of size," said Jeremy E. (Jeb) Brown, chief executive of EPB, whose Philadelphia office was previously the seventh largest in the city. "The two offices combined will be a really potent force in the Philadelphia market. It will solidify our position as a major agency in the mid-Atlantic region."

The move is the eighth merger or acquisition by Bethesda-based Earle Palmer Brown since 1976. EPB, which also has offices in Chicago, Newark, Richmond and Atlanta, is expected to have billings of $250 million this year, according to company officials. Last year, billings for EPB's Washington office were $105 million.

"Our goal is to be one of the major firms in each of those markets," said Brown, who recently hired a mergers and acquisitions specialist and is said to be looking for acquisitions in other areas.

Both EPB and Spiro are full-service communications agencies, and the combined firm will be the largest public relations agency in Philadelphia.

Walter A. Spiro, founder and majority owner of the 29-year-old Philadelphia firm, will be chairman and chief executive of the newly formed company. Spiro is considered one of two dominant figures in the Philadelphia advertising community. The other is Robert G. Wilder, chairman of Lewis, Gilman & Kynett, Philadelphia's largest advertising agency with reported 1986 billings of $113 million.

For several years, the Philadelphia advertising community has wondered what path Spiro, who is in his early 60s, would choose for the company at a time when many independent agencies were merging with or being acquired by larger agencies.

"It's a happy solution," Spiro said yesterday of the acquisition by EPB. "It assures our clients of a competitive ability by being larger and deeper ... and it offers our staff more abundant opportunities."

Spiro has a contract to remain at the agency for five years and then "will start to let the younger people take over," he said.

The wedding of EPB and Spiro also reunites some of the former clients of each agency. In 1985, one of Spiro's major clients, First Pennsylvania Bank, went to EPB/Philadelphia. And, EPB/Philadelphia lost client Strawbridge & Clothier, one of the Philadelphia area's largest and most established department store chains, to Spiro.

Among the combined firm's largest clients will be Omni Hotels, ARA Services Inc., Blue Cross of Greater Philadelphia, New Jersey Transit, Hershey Hotels & Resorts and Arthur Andersen & Co.

EPB is one of the few Washington agencies that competes for national advertising accounts with ad agencies in New York, Chicago and Philadelphia. EPB's national clients include Marriott-owned Big Boy restaurants, Roy Rogers restaurants, USAir, Amtrak and Airbus Industrie.