To discover the components of a trustworthy source, a study evaluated the credibility of health-related news stories. Subjects, 192 college undergraduates, read one of four random versions of a one-page newspaper story about aspirin's ability to ward off heart attacks. They were told that the sources for the articles were: a medical journal (high expertise and trustworthiness), a popular magazine (low expertise, high trustworthiness), and a pharmaceutical company (high expertise, with two versions of varying trustworthiness, claiming aspirin either did or did not lower the risk of heart attacks). Subjects then completed a questionnaire measuring the credibility of the source. Analysis found both expertise and trustworthiness to be valid components of credibility, but that thoroughness, objectivity, and clarity are also factors. Readers were found to judge the medical journal the most thorough and trustworthy source; however, contrary to expectations, the popular magazine was judged no more trustworthy than the aspirin manufacturer. Findings also indicated that the subjects judged the neutral stance of the journal and magazine to be highly credible, while the pharmaceutical company was distrusted regardless of whether it claimed aspirin prevented heart attacks or not. (References and tables of data are appended.) (SKC)