About 30 minutes into trade, the Dow Jones Industrial Average dropped 35.35 points (0.21 percent) to 16,816.49.
The broad-based S&P 500 dipped 0.29 (0.01 percent) to 1,960.67, while the tech-rich Nasdaq Composite Index advanced 3.01 (0.07 percent) to 4,400.94.
Analysts expect light trading volume this week ahead of the US Independence Day holiday, which will close markets early Thursday and all of Friday.
The economic calendar includes reports from the Institute for Supply Management on manufacturing and service-sector activity, as well as US trade deficit figures and, on Thursday, the US jobs report for June.
General Motors was expected to unveil the details of its compensation program for victims of the ignition-recall problem, which has resulted in at least 13 fatalities. GM shares rose 0.4 percent.
Biotech company MannKind jumped 10.5 percent on news that the Food and Drug Administration approved its Afrezza treatment for diabetes.
Bank of New York Mellon gained 3.6 percent following a report that activist investor Trian Fund Management has taken a $1.1 billion stake in the company.
PPG, a supplier of paints, coatings and glass, announced a $2.3 billion acquisition of Mexico's Consorcio Comex, which manufactures coatings in Mexico.
Bond prices were mixed. The yield on the 10-year US Treasury held steady at 2.53 percent, the same level as Friday, while the 30-year dipped to 3.36 percent from 3.37 percent Friday. Bond prices and yields move inversely.