This study examined the effects of message constraints and labels on collaborative argumentation in asynchronous online discussions. Thirty-eight undergraduate students in an introductory educational technology course were assigned to one of three groups. In one group, students posted specific types of messages using a prescribed set of message categories such as argument, evidence, critique, and explanation. Using the same message categories, another group inserted message labels directly into the subject headings to identify each message by category and increase the visibility of the arguments and challenges presented in debates. A control group received none of the above instructions and constraints. Students in the constraints-with-labels group were significantly less likely to (a) challenge other students, and (b) respond to challenges from other students. The label used to identify critiques might have discouraged students from posting critiques and shifted attention to posting more arguments, following up explanations with more explanations, and evidence with more evidence.