Background <p>Identifying determinants of school mental health provider (SMHP) attitudes would illuminate strategies for increasing usage of evidence-based practice (EBP) in schools. SMHP attitudes are influenced by both message content and source influence from supervisors and peers, though the relative importance of these two sources is not clear.</p> Objective <p>The current study experimentally compared the effect of impersonal (i.e., professional organization) and personal (i.e., peer opinion leader and supervisor) source influence on SMHP attitudes and subsequent intentions to use an EBP.</p> Methods <p>SMHPs recruited online were randomly assigned to groups based on the source of information - peer opinion leader, supervisor, or professional organization. SMHPs were asked to imagine that their identified source shared information about the Resilience Builder Program^® (RBP), a transdiagnostic, strengths-based group cognitive behavioral therapy intervention. Conditional process analyses using the SPSS PROCESS macro examined group influence.</p> Results <p>Source credibility significantly predicted perceived acceptability/efficacy, and in turn predicted implementation commitment. Although there were no group differences in credibility and no difference in mediation based on credibility, SMHPs in the supervisor group (relative to the peer group) had more positive attitudes regarding the intervention and higher intentions to use it. Change orientation did not moderate the relation between attitude and implementation commitment, but external community change orientation was positively related to implementation commitment.</p> Conclusions <p>Data from this study support the importance of credibility in shifting attitudes, suggest that supervisors are more impactful than peers through mechanisms separate from credibility, and that external community readiness for change is an important factor.</p>