HumanaStocks of health insurers – including Humana – tumbled Monday as proposed industry mergers received some scrutiny from regulators and an antitrust watchdog.

Shares of Louisville-based Humana fell 2.3 percent. Aetna, the company that wants to buy Humana for $37 billion, slid 1.6 percent. Rivals Anthem and Cigna, which also plan to merge, dropped nearly 3 percent.

The S&P 500 and the Dow Jones posted slight gains.

The American Antitrust Institute on Monday said that it has asked federal regulators to block both mergers because they “would likely harm competition and consumers.”

The AAI said it sent a letter to the U.S. Department of Justice’s Antitrust Division urging the agency to “just say no.”

Also on Monday, the L.A. Times reported that as the state’s insurance regulators have begun public hearings to take a look at the mergers, consumer advocates are voicing concerns.

Anthony Wright, executive director of consumer advocacy group Health Access, told the paper, “While these mergers are in the interest of the insurers, there’s little evidence that they actually benefit patients or our health system as a whole.”

The mergers previously had seen opposition from the American Hospital Association and the American Medical Association.

Aetna and Humana have said the merger would cut costs in part because the combined company would have greater leverage to negotiate for prices with providers, which could reduce costs to consumers.

On Friday after markets closed, Humana also had said it is continuing to struggle with customers it obtained through the Affordable Care Act. Humana said the cost for care for those customers would exceed the premiums it is collecting. The company was still determining the magnitude of the loss.

After markets closed on Monday, Aetna said it expects 2015 earnings to be at the high end of its previously announced range.