When do protest organizations borrow issues or claims that are outside their traditional domains? Sociologists have examined the consequences of borrowing claims across movement boundaries, but not the antecedents of doing so. We argue that movement boundaries are strong when there is consensus about the core claims of a social movement, which we measure by cohesion and focus. Cohesion and focus enhance the legitimacy of a movement and impede member organizations from adopting claims associated with other movements. Analyzing movement organizational activity at U.S.-based protest events from 1960 to 1995, we find that a social movement organization is less likely to adopt claims from other movements when the social movement in which it is embedded exhibits high cohesion and focus. However, when movement organizations do borrow claims, they are more likely to do so by borrowing from movements that themselves exhibit high cohesion and focus. We describe the application of our findings to organization theory, social movements, and field theoretic approaches to understanding social action.