Social media is an increasingly popular tool for disseminating health research findings to members of the general public and may contribute to improving the effectiveness of science communication. This study was designed to investigate how retransmission (i.e., social media content shared by a familiar, credible organization) and modality (i.e., how the message is delivered) influence the effectiveness of communicating health research findings via social media. The findings from a 2 (source) X 3 (modality) X 2 (topic) mixed factorial design experiment (N= 517) indicated that source had a significant effect, such that posts that were retransmitted by a credible organization resulted in greater perceived source credibility, greater perceived message effectiveness, and greater likelihood of an individual to engage with the post on Facebook. Modality significantly increased perceived source credibility and perceived message effectiveness when posts were retransmitted by a credible source, indicating that modality made a difference when messages were elaborated as a function of the retransmission. Also, the topic of the post had a significant impact on the study's dependent variables of interest. Overall, the findings illustrate the potential of retransmission and modality as message features that can improve communication of health research findings on social media. Theoretical and practical implications are discussed.