Stocks find more upward momentum

U.S. stocks opened higher on Monday as shares of energy companies climbed with the price of Brent crude oil and the materials sector benefited from stronger copper prices.

The Dow Jones Industrials gained another 61.03 points, to 18,188.68, following a 168-point gain on Friday.

The S&P 500 added 6.31 points to 2,114.41. The NASDAQ index deducted 8.03 points to 5,018.39.

Stifel downgraded Biogen to hold, stating that the pharmaceutical company has reached a reasonable price target for the next 12 months.

UBS upgraded Staples to buy, noting a favorable risk and reward proposition. The investment bank thinks the office supplies retailer will "more likely than not" be able to close its acquisition of Office Depot.

Jefferies added Pfizer to its "Franchise Picks" list, noting it expects the firm to show significant appreciation over the next one to two years based on better-than-expected launch of breast cancer drug Ibrance. The investment bank also saw potential for more merger and acquisition activity.

Cliffs Natural Resources announced it will sell Chromite assets in Canada to a local mining company, Noront Resources, for $20 million U.S.

Tenet Healthcare and United Surgical Partners International will combine to create the largest provider of ambulatory surgery in the United States in a cash and debt transaction valued at $1.93 billion U.S.

Existing home sales in February were up 1.2%, to an annual rate of 4.88 million units, slightly below expectations. No other economic data is expected Monday.

In the next few days, inflation data, durable goods and the third read on fourth-quarter GDP round out a relatively light week of reports, with few companies reporting earnings.

Prices for 10-year U.S. Treasuries gained slightly, dropping yields to 1.92% from Friday's 1.93%. Treasury prices and yields move in opposite directions.

Oil prices took on 24 cents to $46.81 U.S.

Gold prices dipped $1.90 to $1,182.70 U.S.