Elon Musk canceled the agreement to buy Twitter last Friday.
On Monday, the Internet company hit back and was demonstratively unimpressed by Musk's maneuvers.
A company lawyer wrote in a letter to Musk's lawyer that the "alleged cancellation" of the agreement was "invalid and unlawful": "The agreement is not canceled." to complete the transaction.
Roland Lindner
Business correspondent in New York.
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Musk's lawyer accused Twitter of several violations of clauses in the contract when announcing the withdrawal on Friday.
He cited concerns about the number of "spam" or "fake" accounts on Twitter that are not actually owned by real users.
Twitter says their share of published user numbers is less than 5 percent, Musk claims it could be significantly more.
In the letter, his attorney accused Twitter of making "false and misleading statements" and withholding information from him.
This could result in a "material adverse effect" as described in the contract as the basis for terminating the takeover agreement, i.e. a serious negative change in circumstances.
Twitter was ready to fight as soon as this letter became known and announced that it wanted to go to court.
The company said it was determined to go through with the transaction on the original terms.
Of course, the financial markets are becoming more and more skeptical.
On Monday, Twitter's share price fell 11 percent to $32.65.
He's a long way off the $54.20 deal negotiated with Musk, for a grand total of $44 billion.
In the letter on Monday, Twitter's lawyer wrote that the company had not violated any obligations in the purchase agreement, contrary to what Musk claims.
There is no "material adverse effect".