(FORTUNE Magazine) – So much for the days when we were far from the madding cows. The news, on Dec. 23, that mad cow disease had finally arrived in the U.S. killed the market's holiday spirit. (For more, see "While You Were Out.") Knee-jerk selling drove the indexes down. And trading partners like Japan banned the import of American beef. With the initial madness fading, we looked at companies that stand to reap gains or pain from this bout of temporary market insanity.

Ready to beef up

Immediately after the news broke, the stock of McDonald's (MCD, $26) and other burger joints like Wendy's (WEN, $39) and Jack in the Box (JBX, $23) plunged. But not to worry. Fast-food beef is still what's for dinner. So far, says Morgan Stanley's Mark Wiltamuth, "there's been no impact on fast food sales." Ironically, declines in meat prices from the scare might actually fatten the eateries' bottom line.

Rare in the middle

Fisher Scientific (FSH, $40) saw a 4% jump in its stock price after the news broke, thanks to one division that makes a test for mad cow in Europe. Competitor Bio-Rad (BIO, $58), the leading provider for European tests, rose 20%. Neither company's test has been approved for use in the U.S. Because Fisher doesn't market its own product, it only banks a portion of the profits. So a U.S. market for the test probably won't make its coffers bulge.

Primed for a ribbing

Where's the beef? At Tyson Foods (TSN, $13), which gets almost 50% of its revenue from red meat. The stock took an 8% one-day tumble. Pork producer Smithfield Foods (SFD, $21), which sees about 30% of its revenue from beef, also suffered. The fallout for beef packers? More mad cows could neutralize the demand from Atkins mania. A lot depends on what consumers do starting in January, says Merrill Lynch analyst Leonard Teitelbaum. "The news came during the holidays, so anyone who was going to use roasts had already bought them."