Index added 0.15 point, or 0.01 percent, to 1,380.00. The Nasdaq Composite Index was off 0.62 point, or 0.02 percent, to 2,904.25.
The S&P 500 is still up about 10 percent for 2012, though recent months have eroded those gains. The Nasdaq has fallen for five straight weeks.
Volume was roughly 4.62 billion shares on the New York Stock Exchange, the Nasdaq and the NYSE MKT, well below the year-to-date average daily closing volume of 6.52 billion.
Decliners outnumbered advancers on the NYSE 1,539 to 1,406 on the New York Stock Exchange. On the Nasdaq, decliners outpaced advancers by 1,304 to 1,121.
Merger activity bolstered the price of specific stocks. Precision Castparts Corp offered to buy Titanium Metals Corp for $2.9 billion, while Leucadia National Corp agreed to buy investment bank Jefferies Group for $3.6 billion.
Shares of Titanium surged 42.6 percent to $16.50, while Jefferies climbed 14 percent to $16.27. Precision rose 4.7 percent to $179.46. In contrast, Leucadia fell 3 percent to $21.14.
The S&P 500 dropped more than 2 percent last week, the worst week for the benchmark index since June. The drop was partly propelled by concerns about whether there will be a timely solution to avoid the fiscal cliff.
Gilead Sciences supported the Nasdaq after the company reported over the weekend a 100 percent cure rate using a combination of drugs in a small number of patients with the most common and hardest to treat form of hepatitis C. Gilead was up 13.7 percent at $73.93.
Also in the biotech sector, Celgene Corp rose 5.8 percent to $75.66 after a late-stage clinical trial showed its drug Abraxane improved survival in patients with pancreatic cancer.