This paper studies the role of collateral in credit markets under stress. Australian interbank markets at the time of the Lehman Brothers failure present a platform for identification, because the collateral is liquid and homogenous across borrowers (unlike in retail credit markets), the shock is large and exogenous (unlike in countries with bank failures), and there is comprehensive administrative collateralised and uncollateralised loan-level data. After the exogenous shock, collateralised and uncollateralised borrowing compositions diverge. Uncollateralised borrowing declines for ex ante riskier borrowers while collateralised borrowing increases for borrowers ex ante holding more high-quality collateral. Moreover, riskier banks with sufficient high-quality collateral substitute from uncollateralised to collateralised borrowing. In aggregate, collateralised borrowing expands substantially, predominantly collateralised against second-best (but still high quality) collateral, while interest rates on loans against first-best collateral fall substantially, indicating scarcity of the most-liquid safe assets. This liquid asset demand encourages collateralised lending, contrary to cash hoarding.