Metromedia Inc. will sell its nine-station radio group, including Washington's WASH-FM, for $285 million in the largest radio deal ever, Metromedia said yesterday.

The stations will be bought by an investor group that includes the management of the stations and Morgan Stanley & Co. The price, about 9.5 times estimated 1986 cash flow for the stations, is in line with other recent radio station deals, but does not reflect the premium that Metromedia could have received if it sold the stations one by one.

The sale of the Metromedia stations is the latest in a series of major asset sales by John Kluge, who controls the privately held company. Kluge recently sold his television station group to Rupert Murdoch for about $2 billion. He also recently sold his outdoor advertising business and the Harlem Globe Trotters.

Although Wall Street has been speculating about what Kluge will do with the hundreds of millions of dollars in cash he has amassed, he has made no public statements to suggest that he will journey beyond Metromedia's telecommunications operations. One Wall Street analyst said that Kluge has been rumored to be eyeing certain Hollywood film studios.

Among the properties included in Kluge's latest asset sale were three radio stations in the nation's two biggest markets: WNEW-AM and WNEW-FM in New York and KMET-FM in Los Angeles. The other stations were in Philadelphia, Detroit, Dallas, Tampa-St. Petersburg and Washington.

The radio stations, which are in six of the nation's top 10 markets, reach an estimated 44 million people. The deal also includes the Texas State Network, which provides news and sports in English and Spanish to more than 200 stations.

"Operationally, I don't think this deal will mean any changes," said Metromedia Radio President Carl C. Brazell Jr., who is leading the investor group buying the stations. "I really do believe this is an unprecedented opportunity to buy this many well-established stations in so many of the top markets."

Kluge, who apparently preferred selling the stations to a single buyer even though he might have gotten more by breaking up the group, said he was leaving the radio business reluctantly.

"It is difficult to leave radio after more than 30 years, but we have decided to concentrate on Metromedia's substantial telecommunications business," Kluge said. "The radio stations have enjoyed strong growth under Carl Brazell's leadership, and I am confident that the new company will flourish under the continued direction of Carl and his management team."

Brazell, 45, has worked for Metromedia for 17 years. He said yesterday that he believes $285 million is a "fair price."

Brazell said that Washington's WASH-FM "is an important part of this deal as far as we are concerned. It has a great history in the Washington market, and we feel optimistic that, under present management, it is on its way to being restored to its proper position."

Brazell said WASH-FM's ratings fell several years ago after the music format was altered to appeal to teen-age listeners. He said the station has been "retargeted" to appeal instead to 25- to 49-year-olds.

"Personally, it is very satisfying to be able to acquire this group of stations we have worked so hard to build," Brazell said.

Sharon Ambrust, an analyst with Paul Kagan Associates, described the Metromedia radio stations as "solid performers. I'm curious to see what John Kluge will do with all his money," she added.