Cigna said it filed suit against Anthem demanding a $1.85 billion breakup fee, plus more than $13 billion in additional damages following the February 8 United States court decision blocking the proposed $50 billion merger.
Last week, a federal judge blocked the deal, saying it would lead to higher prices and reduced competition.
Health insurers Aetna and Humana have officially terminated their merger plans, and they did it on Valentine's Day.
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Aetna will pay Humana a $1 billion breakup fee, or $630 million after taxes, and terminated its plan to sell some Medicare Advantage assets to Molina Healthcare Inc, the companies said.
If it passed, the two companies would have combined into an insurance monolith, creating the number one player in the Medicare Advantage marketplace.
For its part, Anthem maintains Cigna does not have the right to cancel the merger.
Last week, Humana released an earnings report that revealed little about whether the company aims to fight on in court to merge with health insurer Aetna or walk away.
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Anthem said Thursday it will appeal the federal court decision blocking its proposed Cigna purchase.
Bloomfield, Connecticut-based Cigna was apparently disappointed in the outcome, and believes the deal has no chance of achieving regulatory approval, despite Anthem's announcement of an appeal. He ruled that the deal would "substantially" reduce competition in the Medicare Advantage market.
But Aetna's attempt to gobble up the nation's fifth largest health insurer brought in the Department of Justice, which sued to block the deal last summer.
Thomas Noland, a spokesman for Humana, said in an email that the company had always known that the deal might not go through.
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The two deals blocked in federal courts would have melded the nation's five largest insurers into three, with UnitedHealth Group Inc. now the biggest.