Abstract. Baltimore City is facing a significant health crisis surrounding rampant use of illicit substances such as opioids. Opioid use disorder (OUD) continues to be a significant public health problem in the city with overdoses continuing to surge. Poor access to care for OUD is contributing to the OUD national crisis, and medications for opioid use disorder (MOUD) such as methadone have been shown to be life-saving medication. Despite the availability of MOUD, there are many challenges in retention in treatment for OUD in urban cities like Baltimore. The parent award (R33DA057747; PI: Magidson) is innovatively designed to improve treatment retention in a Baltimore-based treatment program for OUD using a peer-delivered behavioral intervention (“Peer Activate”). This proposal will supplement the parent award by examining how neighborhood factors and social determinants of health influencetreatment outcomes of OUD and howthese neighborhood factors may moderate the effect of the Peer Activate intervention on retention. The aims of the study include: 1) understand neighborhood-related factors that impact methadone treatment retention; and 2) examine how neighborhood factors may moderate the effectiveness of Peer Activate and how the intervention may be adapted to address these factors in future work. For Aim 1, 30 study participants that received the Peer Activate intervention will be recruited for semi-structuredinterviews, and the interviews will include questions revolving around environmental structures of the neighborhood surrounding the treatment clinic, drop out, and relapse, and how they contribute to low retention in treatment for OUD and how these neighborhood characteristics increase challenges in recovery. Aim2 will use an observational quantitative assessment tool (NIfETy) to collect observational data from the surrounding community, including social disorder (e.g., loitering), physical disorder (e.g., broken windows), physical layout, and violence, alcohol, and drug use indicators. Candidate. This supplement award will support an exceptional candidate, Ms. Anane, who is a first generation Black/AfricanAmerican woman, Flagship Fellow at UMD and third-year public health doctoral candidate. The supplement will support her career goals by gaining skills in research methods that incorporate neighborhood factors and environmental context into clinical trial designs. She will receive additional mentorship and training from expert co-mentors in this area and will attend training workshops and seminars, including GIS for Humanities and Social Sciences and Modern Statistical Learning for Observational Data. The findings from this supplement grant proposal will inform the candidate’s dissertation research and prepare her for a NIH T32 training grant for post-doctoral research. This research will make a unique contribution to understanding the influence of neighborhood-related factors to treatment retention for low-income, largely ethnic/minorit...