Whatever John Malone is up to, it's making Rupert Murdoch -- and Wall Street -- a little nervous.

On Monday, Murdoch's

News Corp.

(NWSWI)

announced an antitakeover plan in response to recent purchases of the company's voting shares by Malone-controlled

Liberty Media

(L) - Get Report

.

It's not clear whether Liberty's ultimate goal is a News Corp. takeover, bargaining chips in any negotiations with management, or simply getting what it sees as a good price on a block of News Corp. shares. A Liberty spokesman didn't reply to a request for comment Monday.

Although a takeover would be audacious, given the Murdoch family's close identification with and control of the media empire, investors didn't seem to like the poison pill move. On Monday, in a reflection of the diminished chances of a takeover, News Corp.'s voting shares -- trading on a when-issued basis on the

NYSE

-- fell 62 cents to $17.58. Non-voting shares dropped 68 cents to $17.08.

Certainly News Corp. -- which last week cleared the last major hurdle in its previously disclosed plan to move from Australia to the U.S. -- is taking Malone's moves seriously. The poison pill was implemented, the company said, after Liberty announced last week it had entered into an arrangement with Merrill Lynch giving it the opportunity to acquire 8% of News Corp.'s voting stock, in addition to the 9.1% Liberty already owns.

"This action was taken without any discussion with, or prior notice to, News Corporation," News Corp. said Monday. "For this and other reasons the company has put in place a rights plan to protect the best interests of all shareholders."

Under the terms of the rights plan, anyone who buys 15% or more of News Corp.'s voting stock -- or adds more than 1% of voting shares to a stake already exceeding 15% -- will find additional purchases of stock rather expensive.

Any such buying, or any launch of a tender offer that would result in ownership exceeding those limits, would give each shareholder a right to purchase a News Corp. share, at half the market price, for each share he owns. In the event of a subsequent merger or acquisition, shareholders would have the right to buy shares of the acquiring entity at half price.

Such measures would make an acquisition of News Corp. impracticably expensive, and cause substantial dilution of current ownership.

The Murdoch family, which will own 29.5% of News Corp.'s voting shares after the incorporation, will be limited, like other shareholders, by the new rights plan. Previously, the Murdoch family had been limited from buying more than 3% of additional voting stock every six months, but News Corp. suggested Monday that the recent Liberty announcement may trigger termination of the Murdoch-specific limitations.

News Corp. says its board will meet within a month with an eye to deciding whether the rights plan should be terminated, amended or kept in place as adopted.

The rights plan will expire on its own within a year; if the board wants to keep the plan alive after that, it will seek stockholder approval.

Absent a shareholder rights plan, Liberty could assemble a huge voting stake in News Corp., as Fulcrum Global Partners analyst Richard Greenfield pointed out last week. If Liberty traded in all its nonvoting shares in News Corp. for voting stock, Malone could assemble a 48% voting stake in Liberty, well above the current Murdoch family stake.

Liberty could try to force News Corp. to buy its 50%, noncontrolling stake in Discovery Communications, speculates Greenfield, or buy all of its Starz/Encore premium cable service.

"We do not believe Mr. Murdoch is pleased with Liberty's maneuvers," wrote Greenfield, who has a buy rating on News Corp.'s nonvoting stock and neutral ratings on News Corp.'s voting stock and Liberty. "We sense relations between News Corp. and Liberty are not what they once were."

The poison pill announcement doesn't detract from News Corp.'s underlying fundamentals, wrote Merrill Lynch analyst Jessica Reif Cohen on Monday, "but will likely impact sentiment until investors become acclimated with this added layer of shareholder/capital structure complexity." Cohen has a buy rating on News Corp.; her firm has done recent banking for News Corp.