A theoretical framework is proposed to test the influence of opinions of other consumers, based on perceived similarity, in two contexts. The perceived similarity of the source may be diagnostic for consumers who are concerned about mispurchase. Experiment 1, therefore, compared the effects of ads featuring a consumer versus an expert as an endorser, with participant use status as a moderator that may affect concerns about mispurchase. Potential product users rated the consumer endorser higher on perceived similarity, which increased ad credibility and resulted in better ad and brand attitudes. Experiment 2 compared the effects of information about a product delivered on a consumer blog with that of information from the advertiser's official Web site, with concern about mispurchase as a moderator. More concerned consumers rated the blogger higher than the advertiser on perceived similarity, leading to greater information credibility and more favorable attitudes toward the blog and the product.