The Dow Jones industrial average shed 5.07 points or 0.03 percent to 14,968.89. The S&P index added 3.08 points or 0.19 percent to close at 1,617.50 Monday after surpassing the 1,600 barrier for the first time on Friday.
The Nasdaq index added 14.34 points or 0.42 percent to 3,392.97.
On the New York Stock Exchange, 1,855 stocks advanced and 1,228 declined on a volume of 3 billion shares traded.
"A couple of months ago everybody was laughing at 1,600. But these [levels] are very important today, because of the individuals on the sidelines. It's new money coming in, which helps push up stocks and to some degree feeds on itself," senior index analyst Howard Silverblatt at S&P Dow Jones Indexes told MarketWatch.
"It's more psychological, that 1,600 points. The same with the Dow at 15,000. They've never made landmarks together," Silverblatt said.
The 10-year treasury note dropped 732 to yield 1.768 percent.
The U.S. dollar rose to 99.38 yen from 99.19 yen. The euro rose to $1.3076 from Friday's $1.3119.
Gold was up, adding $4.60 to $1,468.80 per troy ounce.
West Texas Intermediate crude oil closed higher, adding 15 cents $95.76 per barrel.
On the Chicago Board of Trade, corn for July delivery lost 24 1/2 cents to $6.36 3/4 per bushel. Soybeans for July lost 18 3/4 cents to $13.68 1/2. Wheat for July lost 18 1/2 cents to $7.02 1/2.