Typically, in social movement organizations a relatively small proportion of the membership carries on the organization's principal activities, and the majority participates only marginally. This study examined issue salience, perceived efficacy, and social support as components of activism within the antinuclear war movement. Mail questionnaires were completed by 511 members of two antinuclear war social movement organizations. The findings supported the hypotheses: Salience, personal efficacy, level of social support, and social support from fellow peace workers each predicted a significant (p < .01) independent portion of the variance in activism (multiple R = .62). Longitudinal research is needed to explore the causal role of these components in antinuclear war activism.