Collateral does not flow in a vacuum; it needs balance sheets to move within the financial system. The new regulations constrain private sector bank balance sheets and thus impede market plumbing. This paper looks at securities-lending, derivatives and prime-brokerage markets as suppliers of collateral (as much has been written on the repo market). Going forward, the choice of balance sheet(s), private or public, should be driven by market forces and not by the ad hoc allocation of central banks. Otherwise, this may be suboptimal for monetary policy transmission.