A steep fall in commodity prices pulled down energy and mining stocks for a second day in a row on Monday.
Gold plunged to $1,400 an ounce for the first time since March 2011 as a sell-off in metals continued from last week. Oil prices hit their lowest level since mid-December.
The Dow Jones industrial average was down 89 points at 14,773 shortly after 10 a.m. Monday, a drop of 0.6 percent. Caterpillar led the Dow lower, losing 3 percent to $82.87.
The Standard & Poor's 500 index fell 12 points to 1,577, a loss of 0.7 percent. The Nasdaq composite fell 25 points, or 0.8 percent, to 3,267.
Mining and energy stocks had the biggest losses following the plunge in commodity prices. Freeport-McMorRan Copper Gold and Newmont Mining fell 6 percent, the biggest drops in the S&P 500 index.
Citigroup rose 3 percent to $46.05, one of the biggest gains in the S&P. The bank reported earnings Monday that beat analysts' estimates thanks to strength in investment banking.
Two deals sent some stocks higher. Sprint Nextel jumped after Dish Network offered $25 billion to buy the company. Dish's bid is aimed at beating Japanese phone company SoftBank's offer for Sprint. Dish fell 6 percent to $35.28. Sprint surged 16 percent to $7.20.
Thermo Fisher Scientific offered to pay $13.6 billion to buy genetic testing equipment maker Life Technologies. Thermo Fisher said Monday it agreed to pay $76 in cash for each share of Life Technologies. Both companies' stock jumped. Thermo Fisher rose 3 percent $82.21, and Life Technologies rose 8 percent $73.23.
In the market for U.S. government bonds, the yield on the 10-year Treasury note dipped to 1.71 percent from 1.72 late Friday. The yield remains near its low point of the year, 1.69 percent, which was reached April 5 following news that U.S. employers hired far fewer workers than expected last month.