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Anthem Inc said on Friday it would buy Cigna Corp in a deal valued at $54.2 billion, creating the largest US health insurer by membership.
The deal - the biggest ever in the health insurance industry - comes three weeks after Aetna Inc agreed to buy Humana Inc for $37 billion and is part of an industry-wide consolidation following the roll-out of President Barack Obama's health care law.
Antitrust authorities are expected to aggressively scrutinise how the combinations will affect competition for Medicare and individual and commercial insurance.
By most measures, US insurance markets are extremely concentrated. That means companies could have trouble selling assets if forced to do so by antitrust regulators.
Anthem said it expects the deal to close in the second half of 2016, indicating a long regulatory road ahead.
