The trash talking has begun.

In a stinging letter Friday, garbage hauler Republic Services Inc. rejected an unsolicited $6.19 billion cash offer from larger rival Waste Management Inc., saying the proposal "seriously undervalues" its company.

Republic, the nation's third-largest waste collector, instead wants to stick with its own deal announced last month to buy No. 2 Allied Waste Industries Inc. The all-stock agreement was worth about $6.07 billion at the time.

"Republic is not for sale," James O'Connor, the company's chairman and CEO, wrote to David Steiner, CEO of Houston-based Waste Management, in a letter dated July 18.

O'Connor said the company's board was aware of its fiduciary duties, and that Republic remains off the market because of its prior agreement to buy Allied.

Republic's board of directors believes the Waste Management proposal, announced on Monday, "seriously undervalues" Republic, O'Connor said. Financial analyses presented to officials of Republic and Allied support a valuation "substantially above" $34 a share, he said.

Waste Management's offer on Monday represented a 22 percent premium to Republic's closing stock price of $27.90 on July 11.

But as Republic's shares have climbed on the news this week, that premium has shrunk to 5 percent, based on the company's afternoon trading price of about $32.49 on Friday.

"In your press commentary, you referred to the Waste Management proposal as 'opportunistic,'" O'Connor wrote in his letter. "We believe that your proposal is opportunistic for you and that it will deny Republic stockholders the opportunity provided by the merger between Republic and Allied." Republic is based in Fort Lauderdale, Fla.

Waste Management said in a statement that it is mulling its next step, hinting it might sweeten the deal.

"We are disappointed in the Republic Board of Directors' unwillingness to consider Waste Management's proposal," the company said, adding its offer "could reasonably be expected to lead to a superior proposal," it said.

Brian Butler, an analyst at FBR Capital Markets, said Republic's rejection of the Waste Management offer was expected.

"I think Waste Management knew it was not going to happen at $34," he said.

The choice for Republic to acquire Allied or be bought by Waste Management will come down to money, Butler said.

"I felt that Waste Management can get the deal done in the $38-40 range," he said. "Anything less than $37 you go with Allied Republic," he said. Continued...