The health of 15 of the world's largest financial institutions has been called into serious question, with the Moody's agency downgrading their ratings.
Some of the biggest names in banking, including Bank of America, Citigroup, Goldman Sachs, Barclays, Morgan Stanley, HSBC and Deutsche Bank, have had their ratings slashed.
Others include Royal Bank of Scotland, Credit Suisse, UBS, BNP Paribas, Credit Agricole, Societe Generale, JP Morgan and Royal Bank of Canada.
That will put them under increased scrutiny from markets and lead to potentially higher borrowing costs.
Some of the banks were put on negative outlook, which is a warning that they could be downgraded again later.
Moody's global banking managing director Greg Bauer says all of the banks have significant exposure to the volatility inherent in capital-market activities, and are at risk of outsized losses.