NEW YORK (CNN/Money) - Stocks fell at the open Wednesday, losing footing after two straight days of gains, as investors awaited comments from Fed Chairman Alan Greenspan on the economy and a reading on manufacturing activity in the Chicago region.

At 9:35 a.m. ET, the Dow Jones industrial average (down 51.75 to 8451.24, Charts), the Nasdaq composite (down 6.06 to 1465.24, Charts) and the S&P 500 index (down 4.41 to 913.43, Charts) all retreated. Despite Wednesday's slow start, stocks are still poised to end the month of April with solid gains.

Greenspan was set to weigh in once again on the state of the economy before the House Financial Services Committee, beginning around 10 a.m. ET. The committee said it didn't have enough time to question the Federal Reserve chairman at hearings earlier this year.

Around the same time Greenspan's questioning begins, the National Association of Purchasing Management-Chicago is expected to release its regional index of manufacturing activity for April, expected to edge up to 48.9 from 48.4 in March, according to a consensus of economists surveyed by Reuters. A reading below 50 is considered a signal of contraction.

The Chicago reading could give an indication of what Thursday's national survey from the Institute for Supply Management might show.

In corporate news, a Wall Street Journal report said Tyco International (TYC: Research, Estimates) was likely to disclose roughly $1.2 billion in fresh accounting problems when it announces second-quarter earnings Thursday.

Martha Stewart Living Omnimedia (MSO: down $0.66 to $9.13, Research, Estimates) shed 5 percent in early trading after the home goods company reported a first-quarter loss Wednesday. The company's CEO Martha Stewart is under investigation for alleged insider trading related to her sale of shares of ImClone Systems.

European markets rose at midday, while Asian-Pacific stocks ended mostly higher Wednesday.

Treasury prices fell in early trading, with the 10-year note down 5/32 of a point in price to yield at 3.94 percent. The dollar lost ground against the euro and yen.

Major commodities prices were mixed. Brent crude for June delivery oil dipped 25 cents to $25.24 a barrel in London, having hit a five-month low Tuesday. Gold for June delivery jumped $4.40 an ounce to $338.40 in New York.