I provide the first systematic analysis of collateral choices in one of the main short-term funding markets, the repurchase agreement (repo) market. In repos, long-term bonds serve as collateral connecting short-term and long-term funding markets. In general collateral repos, banks can choose from a list of eligible bonds. Surprisingly, they often deliver more expensive on-the-run bonds rather than cheapest-to-post securities. I rationalize this behaviour using a theoretical model linking the repo and bond markets. My results are relevant for explaining bond market patterns that are different in the euro area compared to the United States.