Among the determinants of social movement success, the characteristics and responses of nonstate organizations under attack by protestors have been overlooked. We examine three campaigns by animal rights groups against experimentation, in 1976–1977, 1987–1988, and 1988–1989. The first two campaigns stopped the research, while the third did not. One influential set of factors was the preexisting vulnerabilities—e.g., unpopular practices, internal factions—on the part of targeted organizations. Another was the strategic responses of these organizations, especially the avoidance of “blunders.” A growing countermovement, thirdly, affected the organizations' ability to respond effectively and avoid blunders. As a social movement expands and strengthens, it encourages counterorganizing and a hardening of resistance, so that many social movements may actually be less successful as they become larger and more visible.