Yahoo (YHOO) shares plunged 19.3 percent after Microsoft (MSFT) CEO Steve Ballmer said the company is no longer interested in buying all of Yahoo, but would be "very open" to a collaboration on Internet search.

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Yahoo (YHOO) shares plunged 21 percent today after Microsoft (MSFT) CEO Steve Ballmer said the company is no longer interested in buying all of Yahoo, but would be “very open” to a collaboration on Internet search.

At the close, Yahoo stock was off $2.41 at $9.14. Microsoft shares were down $1.33, or 6.8 percent, at $18.29 — a 10-year low.

“Let me be clear,” Ballmer said at Microsoft’s annual shareholder meeting this morning in Bellevue. “We are done with all acquisition discussions with Yahoo.”

Yahoo spurned a $47.5 billion takeover offer from Microsoft in May, and later rejected Microsoft’s bid to buy only its search engine. Ballmer has said repeatedly of late that the buyout remains off the table, though a search-related deal is possible.

But today marked the first time he had renewed that stance since the resignation announced this week by Yahoo co-founder and CEO Jerry Yang, who had resisted Microsoft’s overtures. Yahoo shares rose 8.7 percent after Yang said he would step aside, because investors hoped it meant a deal with Microsoft would now be more likely.

Ballmer said the companies are not currently talking about a search deal.

Some analysts have interpreted Ballmer’s public comments about a Yahoo buyout as negotiating posturing, and suspect Microsoft might still want to grab Yahoo at a low price, in hopes of improving their joint position in online search and advertising. However, analysts have also said Microsoft is likely to wait until next year before deciding, giving it time to watch Yahoo’s performance and study the antitrust regulatory climate in a new administration in Washington.