Although social movement organizations (SMOs) are often depicted as mobilizing intensive resources from their individual members, we lack a systematic assessment of this issue. Based on the notion of ‘modern social movements’ I argue that SMOs mobilize fewer human and monetary resources from their members than other voluntary organizations do. A regression model using a survey representative of American adults shows that, when compared to other organization members, SMO members generally contribute significantly lower amounts of money and time. They are also less likely to attend, plan or chair meetings, and give speeches on behalf of the organization. The only exception to this pattern is that SMO members are more likely to write letters for the organization than other members do.