The Standard & Poor's 500 index fell 15 points, or 0.8 per cent, to 1,856 as of noon (1600 GMT) Thursday.
The Dow Jones industrial average lost 75 points, or 0.5 per cent, to 16,361. The Nasdaq composite index dropped 75 points, or 1.8 per cent, to 4,108.
Ebay dropped 3 per cent on news that Carl Icahn is backing down from his campaign to shake up the company.
Biotechnology companies fell sharply. The sector has become volatile in recent weeks amid scrutiny over the cost of their drugs.
Bond prices rose. The yield on the 10-year Treasury note fell to 2.63 per cent.