US stocks have mostly fallen but the Nasdaq stayed in positive territory as Israel's Teva announced a $US40.1 billion ($A51.92 billion) cash-and-stock bid for generic drug maker Mylan.

US stocks have mostly fallen but the Nasdaq stayed in positive territory as Israel's Teva announced a $US40.1 billion ($A51.92 billion) cash-and-stock bid for generic drug maker Mylan.