Project Summary/Abstract Despite the prevention and treatment efforts of the past 30 years, Latino men who have sex with men (MSM) continue to be disproportionately impacted by HIV: diagnoses increased 14% between 2010-2014 and those younger than 40 years old had the highest incidence rates. HIV biomedical interventions to reduce HIV infections are not reaching Latino communities. Nearly 300,000 Latinos in the nation were eligible for HIV pre- exposure prophylaxis (PrEP) in 2015, yet only 7,600 received a prescription for it. Yet a powerful cultural source of motivation for behavior change has been underutilized: the family. Latinos report a high reliance on family for material and emotional support and often choose healthy behaviors to “do right” by their family. Yet no family-based interventions to promote PrEP have been tested among Latino MSM at high-risk for HIV infection. Although Latino MSM often experience family rejection and stigma, studies show that many continue to prioritize family support for behavior change over other sources of support; that strained family ties are reestablished over time; and that Latino MSM and their siblings come to rely on each other as they age. Siblings are often a lifelong source of social support and 77% of Latinos have at least one sibling. This project aims to engage the siblings of Latino MSM in the development and delivery of PrEP-uptake messages and to evaluate the initial efficacy of involving siblings in HIV biomedical interventions. Specific Aims: (1) To develop intervention messages to increase PrEP use in high-risk Latino MSM that can be delivered by their siblings. We will use IMB model constructs to create these messages. We will conduct in-depth interviews with 30 LMSM-Sibling pairs to identify factors relevant for the development and delivery of messages to increase PrEP-use. Messages will be rated by a panel of experts and tested with 3 focus groups. (2) To implement the intervention messages in an uncontrolled pilot study with Latino MSM and their siblings (n=20 pairs). We will use a pre/post test design with 30-day follow-up with 20 Latino MSM-sibling pairs to gather feasibility and acceptability data, and to refine messages and delivery procedures as needed. (3) To conduct a two-group randomized control trial (n=124) to evaluate the efficacy of sibling-delivered PrEP messages compared with provider-delivered standard PrEP messages for use with high-risk Latino MSM. An important aim of this project is to develop an effective and brief intervention that can be sustained by our community partner and PrEP Center for Excellence, St. John’s Well Child and Family Center, and ultimately scaled-up by health departments and other PrEP stakeholders. Future studies could adapt this approach for other racial/ethnic sexual and gender minorities.