Inoculation messages employed in past studies have been consistently preventative. Yet, if inoculation strategies are to be used in mass media campaigns, researchers need to know what the effects will be on all audience members—not just those known to support a message sponsor’s position. A 3-phase experiment was conducted involving 558 participants. Linear regression analyses identified that initially supportive, neutral, and opposed subjects exposed to the inoculation message reported significantly more positive attitudes toward the study topic of agricultural biotechnology following an attack message than did their respective controls. The inoculation message contributed to significantly increased threat levels among initially neutral and opposed subjects and marginally increased counterarguing output among initially supportive and neutral subjects. Additionally, counterarguing partially mediated final attitudes for inoculated supportive subjects.