Purpose The purpose of this paper is to investigate how the source of brand-related user-generated content (UGC) (a close friend vs a celebrity) interacts with content sponsorship (organic UGC vs sponsored UGC) to influence consumer causal attributions, brand attitude, and intention to comply with the recommendation. Design/methodology/approach In all, 285 college students participated in a two (source: a close friend vs a celebrity) by two (content sponsorship: organic vs sponsored) between-subjects online experimental design. Findings Results showed that recommendation from a close friend generated more information-sharing attributions and less monetary-gain attributions than did recommendation from a celebrity when the brand-related UGC was organic. In contrast, source type did not influence causal attributions differently when the UGC was sponsored. Further, this study demonstrated that both information-sharing and monetary-gain attributions mediated the effects of source type and content sponsorship on brand attitude and intention to comply with the recommendation. Originality/value This study is one of the first to examine the effectiveness of celebrities as a source of brand-related UGC. Also, this research extends the existing knowledge about source effects by examining the relative effectiveness of two sources of product information, close friends and celebrities, who have both been found to be individually effective in the traditional marketing context. Additionally, the findings of this study that the relative effectiveness of source type depends on whether brand-related UGC is sponsored or not add a further insight into how source type influences the effectiveness of brand-related UGC.