A growing number of companies are going hostile.

With many stock prices well off their highs, a number of companies are trying to seize an unusual opportunity to buy their competitors on the cheap. However, in many cases, the targets are not caving easily.

The latest example: Earlier this week, Thermo Fisher Scientific, which makes laboratory equipment, offered $6 billion to buy Millipore, which would increase its presence in the manufacture of biotech drugs, according to Reuters.

Late Tuesday evening, Millipore confirmed its Board of Directors is evaluating "strategic alternatives to enhance shareholder value, including by pursuing a process with potential bidders to explore a possible merger or sale of the company." It did not identify Thermo Fisher by name.

Millipore stressed that in the end there may not be a transaction.

In a more contentious battle, last Tuesday Simon Property, the largest real estate investment trust, said it offered to buy bankrupt REIT General Growth Properties in a deal valued at more than $10 billion. The unusual deal would provide 100 percent cash recovery of par value plus accrued interest and dividends to all General Growth unsecured creditors, the holders of its trust preferred securities, the lenders under its credit facility, and other creditors.

"Full cash payment to all unsecured creditors and the substantial recovery for equity holders that Simon has proposed would be a great result," said Michael Stamer, counsel for the Official Committee of General Growth's Unsecured Creditors, in a statement.

General Growth, however, was not as enthusiastic about the offer. It fired off a letter to Simon asserting that the offer was "not sufficient" to preempt its process to explore all avenues to emerge from Chapter 11 and maximize value for all the company's stakeholders.

Meanwhile, Brookfield Asset Management, another real estate company based in Canada, is putting together its own offer for a large piece of General Growth, according to published reports. However, no deal has been announced.

Also on Monday, for the second time this month Airgas rejected an unsolicited takeover offer from rival Air Products, which offered $60 per share in an all-cash deal.

"The Airgas Board of Directors is unanimous in its belief that the Air Products offer significantly undervalues Airgas and fails to reflect the value of our industry leading position and future growth prospects," said Airgas Chairman and CEO Peter McCausland, in a press release.  

In its response, Air Products asserted that its offer "would also create more value for Airgas shareholders than it can achieve on its own."

You can bet this battle will get uglier before it is resolved.

Meanwhile, for more than a year Agrium has made several offers to buy rival fertilizer company CF Industries Holdings. On Feb. 19, Agrium said it would extend its latest offer of $45 in cash plus one Agrium share per CF share until March 22.

CF, however, has been able to hold out because it has a poison pill.

Agrium, however, also has a Plan B. In January, Agrium said it would nominate two independent directors for election to CF's board at CF's 2010 annual meeting. Agrium also said it plans to seek CF stockholder approval for a resolution requesting that the CF board redeem CF's poison pill.

"We remain fully committed to acquiring CF and believe the CF board of directors has failed to uphold its stockholders' interests by continuing to ignore Agrium's strategically and financially compelling offer, currently valued at $110.32 per CF share," said Mike Wilson, Agrium's President and CEO, in a statement. "We believe CF stockholders are entitled to the opportunity to benefit from our offer which they have consistently supported."

Although hostile deals frequently attract a lot of media attention, they represent a fraction of the M&A deals. According to Reuters, hostile bids represented only 0.9 percent of the M&A market in 2009.

However, it noted that this was the highest rate of the past five years.

Dealogic, on the other hand, says global hostile bid volume was down 66 percent from a year earlier.

In any case, some experts expect more hostile deals this year due to the continued wide disparity between public valuations of companies and what the potential targets think their companies are worth.

Of course, the reduction in bank financing could hinder some activity. Still, making a hostile bid still makes sense for companies that can make either an all-cash bid or can offer a combination of cash and stock to complete a deal.