The Dow Jones industrial average is up 31 points to 16,095 Monday. The Standard & Poor's 500 index has gained one point to 1,806. The Nasdaq is up six points to 3,997.
The Nasdaq briefly crossed the 4,000-point mark, a level it hasn't seen since the dot-com bubble.
Sunday's international deal with Iran was the first significant progress in years to curtail that country's nuclear ambitions. It could reduce the risk of conflict, improve trade and boost oil supplies.
The price of benchmark U.S. crude fell $1.04 to $93.80 a barrel.
Energy was the biggest decliner among S&P 500 industry groups, due to lower oil prices.