Wall Street bankers have reacted with scorn to sweeping new rules proposed by President Barack Obama to curb their activities.
The Financial Services Roundtable, which represents large Wall Street institutions, says the president is trying to roll back the clock.
"The better answer is to modernise the regulatory framework and not take the industry and the economy back to the 1930s," it says.
But Mr Obama says he's ready to press ahead. "Never again will the American taxpayer be held hostage by banks that are too big to fail," he says.
He says that banks safeguarded against failure by taxpayers should not take unnecessary risks, and that he will close loopholes allowing those banks to undertake risky transactions for their own profit.
Under the proposals, which require congressional approval, banks - or financial institutions that own banks - will be banned from owning, sponsoring or investing in a hedge fund or a private equity firm.
They will also be barred from proprietary trading operations - in which a firm bets on financial markets with its own money rather than trading for a client.
Not with taxpayers' money you don't - Obama
"If financial firms want to trade for profit," Mr Obama says, "that's something they're free to do. But these firms should not be allowed to run these hedge funds and private equity funds while running a bank backed by the American people."
The reform aims to restore the distinction between investment banking and commercial banking activities - a distinction abolished in 1999.
Share prices dropped sharply in response to the announcement. The Dow Jones closed down 2% - its worst fall since October - while Japan's Nikkei closed at a three-week low, down 2.59%.
Shares in major US banks Goldman Sachs and Bank of America all fell.