In February, we reported on a major deal between Mt. Gox, the Japanese firm that runs the world's largest Bitcoin exchange, and the American startup CoinLab. Under the terms of the agreement, CoinLab would take charge of Mt. Gox's operations in the United States and Canada. CoinLab promised that its connections to American banks and investors would allow it to provide a superior customer experience.

But that partnership has collapsed with a lawsuit first reported by Gawker. CoinLab sued Mt. Gox on Thursday, charging that the Japanese firm failed to live up to its side of the contract that the two firms signed in November. Under the terms of the contract, Mt. Gox was supposed to hand off its North American operations to CoinLab in March.

But CoinLab says that didn't happen. "Mt. Gox has continued to market to customers in North America and has accepted business from customers there," CoinLab's lawsuit, filed in Washington State on Thursday, says. "Mt. Gox has also failed to provide CoinLab with account reconciliation data, server access, and other information promised in the agreement that is essential for CoinLab to market exchange services and service its customers as contemplated in the agreement."

The contract specifies that "liquidated damages" for a breach of the contract by either party would be $50 million. CoinLab says it has suffered actual damages that are even larger, though: $75 million.

The lawsuit is the latest bad news for Mt. Gox, which has suffered a series of technical problems in recent months. The site was overloaded for much of April 10, the day Bitcoin reached an all-time high of $266 before losing more than half of its value. Mt. Gox faced further problems in the weeks that followed as the site became the target of denial-of-service attacks.

"Our hope is that we can quickly resolve the issues and clear a path to better serve our US and Canadian customers," CoinLab CEO Peter Vessenes said in a statement posted to the company's website. "While it’s disappointing to file a lawsuit, we can rest assured that we tried every other avenue to resolve the issues outside of the courtroom."

Mt. Gox founder Mark Karpeles declined to comment to Gawker because he hadn't yet had time to review CoinLab's lawsuit. He didn't respond to our request for comment.

Disclosure: I own some Bitcoins.