Poison pill” is a term that conjures memories of the 1980s, when corporate raiders such as Carl Icahn and Kirk Kerkorian acquired companies through hostile takeovers.

But last year’s market crash, which depressed stock prices across the board, has many public companies rethinking the benefits of beefing up their takeover defenses.

“We’ve seen more poison pills adopted recently than we had for the past several years,” said Dean Salter, a partner at Holme Roberts & Owen LLP in Denver, Colorado. “They’d gone out of favor six or eight years ago.”

A total of 51 hostile takeovers or unsolicited bids were initiated in the United States during the first half of 2009, up from 38 in the same period of 2008, according to Dealogic, a New York-based data provider. So far this year, 61 hostile or unsolicited bids have been made, compared with 79 in all of 2008.

With share prices down, publicly traded companies are more exposed.

“In significant part, it’s a function of most companies’ equity market cap being lower than it used to be, so people feel vulnerable,” said John Elofson, an attorney with Davis Graham & Stubbs LLP in Denver. “Then there’s the derivative usage by activists and would-be raiders, which means that companies may have less warning of a potential hostile acquisition attempt than they might have in years past. Those things combined are making people more sensitive to what their profile is.”

The danger of derivatives came to light several years ago, when hedge funds TCI and 3G Capital Partners amassed a 12 percent ownership stake in Jacksonville, Florida-based railroad giant CSX Corp. by buying swaps through investment banks. Because they didn’t own the stock outright, they avoided the 13D rule, which requires investors to disclose their position when they’ve acquired 5 percent or more of a company’s shares.

Thus, they were able to keep their interest in CSX secret from other investors and avoid triggering a run-up in CSX stock.

The hedge funds argued that they couldn’t tell their investment banks—the shares’ legal owners—how to vote, an argument the judge found dubious.

“One may suppose that banks seeking to attract swap business well understand that activist investors will consider them to be more attractive counterparties if they vote in favor of the positions their clients advocate,” U.S. District Judge Lewis Kaplan wrote when he ruled against the hedge funds in June 2008.

Some lawyers doubt that poison pills will make a big comeback.

“I’ve seen companies adopting poison pills in the oil patch and energy-related areas, but I’m not seeing it so much elsewhere,” said Marc Musyl, an attorney at Greenberg Traurig LLP in Denver.

The current regulatory environment is trending toward more shareholder input in company decisions, not less, he noted.

“The current regulatory environment frowns upon anything that diminishes shareholders’ rights at the potential expense of management,” Musyl said. “On the other hand, yes, I believe there is a lot more takeover activity going on, both hostile and nonhostile.”