Bear Stearns and Lehman Brothers were two large US investment banks before the financial crisis. Due to their high exposure to the subprime mortgage market, their financial positions were unsustainable by March and September 2008, respectively.
Bear Stearns was bought by JP Morgan Chase & Co. at a 94% discount for 236 USDmn in loan backed by the US Federal Reserve (central bank) whereas Lehman Brothers was not bailed out at all. Estimates of the needed size of the Lehman bailout were around 50 USDbn. The collapse of these two institutions contributed to the start of the Global Financial Crisis.