Those concerns offset an encouraging pickup in hiring by U.S. employers last month.
The Standard & Poor's 500 index fell a point to 1,875 as of noon Eastern time.
The Dow Jones industrial average rose 27 points, or 0.2 per cent, to 16,450. The Nasdaq composite lost 20 points, or 0.5 per cent, to 4,332.
Biotechnology and health care stocks were among the biggest decliners.
Bond prices fell. The yield on the 10-year Treasury note rose to 2.80 per cent from 2.74 per cent late Thursday. The yield had been as low as 2.60 per cent earlier this week.