We study the role of political collective action in shaping subsequent social interactions. In an experiment, subjects choose whether to sign a petition, or report whether they participated in recent street protests. Before and after, subjects are put in pairs to interact in games that measure prosocial preferences. Following participation choices, we observe increased prosociality between participants, but not within other pairs. Our structural estimation recovers individual prosocial preferences, showing that they increase as a result of joint participation. We then show that participating individuals receive private payoffs in subsequent interactions with fellow participants. Because of this, expecting higher participation by peers makes an individual more likely to participate. This mechanism suggests a reason why citizens participate in political collective action, and helps explain the role of coordination and signalling.