Since the advent of the Internet, word of mouth (WOM) has been integrated into the online channel, with users writing their opinions and reading reviews from other members. Currently, WOM has been growing and developing on social network sites, including Instagram. The figure of influencer has emerged as especially prominent on this particular social media platform. Brands have become aware of the importance of influencers and contact them to publish brand content to their profiles. This has become a new marketing strategy in which companies can reach greater numbers of consumers and exert a significant influence on them. Against this background, the current research analyzes how brand posts and stories by influencers on Instagram affect consumers, depending on the format used (post vs. story) and whether the content is promoted. A 2x2 experimental study is conducted with 573 participants. Results show that the content published by influencers affects consumers’ purchase intention differently depending on the format, wherein stories are more persuasive than posts. However, we find that indicating that the publication is promoted has no impact on consumers.