BackgroundMen who have sex with men (MSM) in the United States experience a disproportionate burden of HIV and bacterial sexually transmitted infections (STIs), such as gonorrhea and chlamydia. Screening levels among MSM remain inadequate owing to barriers to testing such as stigma, privacy and confidentiality concerns, transportation issues, insufficient clinic time, and limited access to health care. Self-collection of specimens at home and their return by mail for HIV and bacterial STI testing, as well as pre-exposure prophylaxis (PrEP) adherence monitoring, could be a resource-efficient option that might mitigate some of these barriers.ObjectiveProject Caboodle! is a mixed-methods study that explores the acceptability and feasibility of self-collecting and returning a bundle of 5 different specimens for HIV and bacterial STI testing, as well as PrEP adherence monitoring, among sexually active HIV-negative or unknown status MSM in the United States aged 18 to 34 years.MethodsParticipants will be recruited using age, race, and ethnicity varied advertising on social networking websites and mobile gay dating apps. In Phase 1, we will send 100 participants a box containing materials for self-collecting and potentially returning a finger-stick blood sample (for HIV testing), pharyngeal swab, rectal swab, and urine specimen (for gonorrhea and chlamydia testing), and hair sample (to assess adequacy for potential PrEP adherence monitoring). Specimen return will not be incentivized, and participants can choose to mail back all, some, or none of the specimens. Test results will be delivered back to participants by trained counselors over the phone. In Phase 2, we will conduct individual in-depth interviews using a video-based teleconferencing software (VSee) with 32 participants from Phase 1 (half who returned all specimens and half who returned some or no specimens) to examine attitudes toward and barriers to completing various study activities.ResultsProject Caboodle! was funded in May 2018, and participant recruitment began in March 2019. The processes of designing a study logo, creating advertisements, programming Web-based surveys, and finalizing step-by-step written instructions accompanied by color images for specimen self-collection have been completed. The boxes containing 5 self-collection kits affixed with unique identification stickers are being assembled, and shipping procedures (for mailing out boxes to participants and for specimen return by participants using prepaid shipping envelopes) and payment procedures for completing the surveys and in-depth interviews are being finalized.ConclusionsSelf-collection of biological specimens at home and their return by mail for HIV and bacterial STI testing, as well as PrEP adherence monitoring, might offer a practical and convenient solution to improve comprehensive prevention efforts for high-risk MSM. The potentially reduced time, expense, and travel associated with this approach could facilitate a wider implementation of screening algorithms and remote monitoring strategies.International Registered Report Identifier (IRRID)PRR1-10.2196/13647