Abstract The effective liquidity supply of the economy—the weighted-sum of all assets that serve as media of exchange—matters for interest rates and unemployment. We formalize this idea by adding an over-the-counter market with collateralized trades to the Mortensen–Pissarides model. An increase in public liquidity through a higher supply of real government bonds raises the real interest rate, crowding out private liquidity and increasing unemployment. If unemployment is inefficiently high, keeping liquidity scarce can be socially optimal. A liquidity crisis affecting the acceptability of private assets as collateral widens the rate-of-return difference between private and public liquidity, also increasing unemployment.