Project Abstract Assisted partner notification services (APS) involves public health contract tracing for clients with HIV or other sexually transmitted infections, in order to confidentially notify contacts of their exposure and link them to testing and treatment services. Through several randomized controlled trials (RCTs), APS has been demonstrated as a cost-effective strategy for increasing HIV testing and case identification. Some APS programs implemented at scale in low- and middle-income countries have fallen short of optimal outcomes, because APS participants typically only name their current sex partner or fail to notify their partners. Finding innovative ways to increase the number of partners elicited and the number completing HIV testing is critical to success in scale- up. In Ukraine, HIV prevalence is estimated at 1% among adults, 7.5% among men who have sex with men (MSM), and 22.6% among persons who inject drugs (PWID). There is a substantial undiagnosed fraction of people living with HIV, estimated at 25% overall, 41% among MSM, and 42% among PWID. Ukraine scaled up APS in 40 public-sector health facilities in 2019-21; however, the contact index (number of partner contacts named per index client) was only 1.14 for sexual and needle-sharing partners, and 23.3% of partners with unknown status did not complete The CASI-Plus mHealth tool leverages computer-assisted self-interviews (CASI) in 2 steps: 1) an initial contact elicitation interview designed to reduce social desirability bias associated with reporting multiple sexual partners, same-sex sexual partners, or needle-sharing partners; and 2) repeated client follow-up surveys and health worker engagement prompts to facilitate communication and case management through the partner notification and testing process. conduct formative research from health worker and client perspectives to design the CASI-Plus mHealth intervention assess adoption of CASI-Plus and its impact on contact elicitation CASI-Plus assess appropriateness and feasibility of the CASI-Plus intervention and its impact on overall acceptability of APS services. testing. The study’s specific aims are to: 1) (R21) ; and 2) (R21) in a pilot-level RCT, ; 3) (R33) test on the HIV testing index in an expanded RCT; and 4) (R33) The research team includes experts in APS programs, digital health development, usability assessment, and mixed methods health services research from the University of Washington and the Ukraine Ministry of Health’s Public Health Center. This partnership offers great potential to integrate the mHealth solution into the national health information system and translate research findings into policy and practice, should acceptability, appropriateness, feasibility and effectiveness be strong.