Abstract: Through two experiments, this study assessed source and message effects of Instagram-based pro-veganism messages. Experiment 1 (N = 294) examined effects of organization (brand versus non-profit) and message types (egoistic versus altruistic) on consumer responses to Instagram-based pro-veganism content. Experiment 2 (N = 288) examined effects of source type (celebrity versus non-celebrity) and message valence (positive versus negative)on consumer responses to Instagram-based pro-veganism content. Results demonstrated significant main effects of organization type, with consumers indicating more positive attitudes and higher credibility towards the brand. Significant main effects of message type were also found, with altruistic messages eliciting higher perceived information value than egoistic messages. Subjective norms had moderating effects on attitude towards the organization, while attitude towards veganism had moderating effects on perceived information value. Results also indicated significant main effects of message valence on perceived information value of pro-veganism Instagram posts and significant interaction effects of the two manipulated factors on intention to spread electronic word-of-mouth (eWOM) about veganism. Implications for use of Instagram-based health marketing communication about veganism were discussed. Specifically, organizations looking to use social media to influence attitudes and behavioral intentions towards health issues should seek to reach their target audiences through selecting endorsers and messages that will optimally present the health issue in a relatable and engaging way.