Following a recommendation by Transparency International, we conduct a laboratory experiment to gauge the impact of a specific type of grassroots participation on petty corruption. Participants play a one-shot, three-person sequential bribery game that, depending on the treatment, either gives or does not give passive third parties suffering from corruption the opportunity to send a publicly visible message to potential bribers and bribees. We find that messaging opportunities deter bribe offers (i.e., the extensive margin of bribe), but affect neither the size of the offered bribe (i.e., the intensive margin) nor bribe acceptances. We conjecture that the different impact of the treatment on bribe-givers and bribe-takers may be due to the order of play.