Objective: Evidence of the health and environmental harms of red meat is growing, yet little is known about which harms may be most impactful to include in meat reduction messages. This study examined which harms consumers are most aware of and which most discourage them from wanting to eat red meat. Design: Within-subjects randomized experiment. Participants responded to questions about their awareness of, and perceived discouragement in response to, eight health and eight environmental harms of red meat presented in random order. Discouragement was assessed on a 1-to-5 Likert-type scale. Setting: Online survey. Participants: 544 US parents. Results: A minority of participants reported awareness that red meat contributes to health harms (ranging from 8% awareness for prostate cancer to 28% for heart disease) or environmental harms (ranging from 13% for water shortages and deforestation to 22% for climate change). Among specific harms, heart disease elicited the most discouragement (mean=2.82 out of 5), followed by early death (mean=2.79) and plants and animals going extinct (mean=2.75), though most harms elicited similar discouragement (range of means, 2.60 to 2.82). In multivariable analyses, participants who were younger, identified as Black, identified as politically liberal, had higher general perceptions that red meat is bad for health, and had higher usual red meat consumption reported being more discouraged from wanting to eat red meat in response to health and environmental harms (all p<0.05).