After MF Global went bust, most people believe it was an extreme "spectacular recklessness" under Jon Corzine, and that the U.S. banks should have only "moderate" European Exposure.  However, banking stocks have been under pressure with increasing investors worries. 

Jefferies Group, for example, eventually disclosed detail position it held on European debt earlier this month after its shares plunged more than 20%.  But other banks have not followed suit asBloombergnotes that since it is not required by the U.S. regulation,

"Firms including Goldman Sachs and JPMorgan don't provide a full picture of potential losses and gains in the event of a European default, giving only net numbers or excluding some derivatives altogether." 

U.S. stocks took a beating after Fitch Ratings said on Wed. Nov. 16 that Europe’s debt crisis may pose a “serious risk” to U.S. banks, driving investors to safer bets such as U.S. Treasurys.  Fitch also notes that although U.S. banks have been reducing their direct exposure for well over a year, but they haven't clearly disclosed the extent of their holdings of European sovereign debt or their trading positions with European counterparties.

There are clues to somewhat quantify the potential exposure on a global basis and of the U.S. banks.