The bicameral nature of most U.S. legislatures implies that it is often in the interest of legislators to collaborate on legislation across chamber lines. In this research, we offer a novel theory of collaboration between upper and lower chamber members in U.S. legislatures. We expect that exogenous characteristics like party, constituency, and joint committee membership characterize collaboration across legislative chambers. Additionally, we expect that endogenous patterns of reciprocity will also characterize choices about cross-chamber collaboration. Using data on cross-chamber bill sponsorship in legislatures in Texas, Colorado, Maine, and Oklahoma, empirical evidence largely corroborates our expectations. Cross-chamber collaborative choices are reciprocal, and reciprocity occurs both within and between parties.