The Washington Post Co. yesterday announced that it had agreed to buy virtually all of Capital Cities Communications' cable television properties for $350 million in cash.

The acquisition, which has been expected, is the largest in the Post Co.'s history and would make it the 20th-largest cable systems operator in the United States.

"This acquisition gives us a major stake in cable television," said Post Chairman Katharine Graham in a statement. "After more than a decade of substantial investment, we believe the industry is in a position to deliver the kind of returns that have been forecast for years."

The 53 cable systems, which serve roughly 350,000 subscribers in 15 states, are being sold so that Capital Cities can meet the cross-ownership-of-media provisions that govern the company's pending merger with American Broadcasting Cos. Federal Communications Commission regulations prohibit the ownership of both a television network and major cable systems.

The Capital Cities cable systems in Plymouth and Saline, Mich., will not be included in the deal because they overlap with the Washington Post Co.'s ownership of WDIV-TV, the NBC affiliate in Detroit. FCC regulations also prohibit cross-ownership of media in the same market.

The $350 million price tag means the Post will be paying approximately $1,000 per Capital City cable system subscriber, a price analysts consider reasonable.

"It's a fair deal for both sides," said Drexel Burnham media analyst John Reidy. "The deal to us looks right in line with other cable deals, maybe even a shade cheaper."

"It's a good deal for the Washington Post," said cable analyst Paul Kagan, "It's hard to find a group of systems like this for a thousand dollars a subscriber or less. I think there are other cable companies that would have paid more money for that system if they had been given the opportunity to bid on them."

Washington Post Co. President Richard Simmons declined to comment on whether the company had to bid for the properties against other potential buyers. Analysts pointed out that, unlike other Capital Cities properties sold through a corporate auction process conducted by investment banker Goldman Sachs, the cable sale appeared to be the result of a negotiation by the Post Co. and Capital Cities.

Unlike many other cable companies actively bidding for franchises and building systems, the Capital Cities systems are relatively mature and currently profitable. Revenue for the systems exceeded $76 million in 1984 with more than $25.5 million in cash flow generated. The operating cash flow margin -- revenue after depreciation and other charges -- was higher than 33.6 percent, one of the highest in the cable industry.

"This is the time to get into cable," said Simmons. "I think prices for cable systems have not yet fully reflected the new pricing flexibility that will soon be available."

Simmons and Wall Street analysts point to federal legislation passed last year that effectively will free local cable systems from municipal rate regulation in 1987, thus enabling them to charge more for basic cable service.

"We would obviously expect to benefit from the cable deregulation act," said Simmons, who described it as a "key variable" in the Post's decision to purchase the systems.

Much of the future revenue and profit growth from the systems could come from this new pricing flexibility, Simmons said.

Ultimately, the Post may consider "swapping" cable systems with other cable companies to "cluster" cable systems to gain geographical economies of scale, he said. Similarly, the company may seek to obtain cable systems in the Northeast, where the Post's Sportsvision pay cable television programming service could be more effectively marketed.

The $350 million purchase price should dilute Post earnings by roughly $1.85 a share in the first year, according to the Post Co., which earned $6.11 a share in 1984. The Post Co. also owns four television stations, Newsweek magazine, The Washington Post and Stanley Kaplan educational centers.

Post stock closed up 1 1/2 to 117 yesterday on trading of 16,700 shares.