Getty Images / Tim Boyle
Near 9:34 a.m. ET, the Dow was down 66 points, the S&P 500 was down 8 points, and the Nasdaq was down 11 points.
The indexes closed out last week with small gains, supported by a surge in stocks on Friday after the November jobs report came in better than expected. The US economy added 211,000 jobs, and the unemployment rate was unchanged at 5%, a seven-year low.
Meanwhile, crude oil was still falling on Monday morning, following OPEC's indecision on Friday on a production quota, and the lifting of its output to 31.5 million barrels per day.
Brent crude collapsed 2% to a six-year low of about $41.78 per barrel, while West Texas Intermediate crude futures in New York fell about 3% to as low as $38.50, less than $1 from the year-to-date low of $37.75.
The energy sector was the biggest loser on the S&P 500 in early trading. Chevron was down 3%, the most among components of the Dow Industrial Average.
The economic data calendar is light today and throughout this week. The Fed's Labor Market Conditions Index is due at 10:00 a.m. ET, and consumer credit data at 3:00 p.m.
Keurig Green Mountain shares popped 74% on news that the company is going private, and will be acquired by JAB Group for $92 per share. The price is a 78% premium from Keurig's closing level on Friday.