PROJECT ABSTRACT In the U.S., over 400,000 tobacco users call state-funded quitlines each year for help with quitting smoking. A recent study found that 25% of those who call tobacco quitlines reported using marijuana/cannabis and 44% of those were interested in quitting or cutting back on their marijuana (MJ) use (in addition to wanting to quit tobacco). Unknown is how best to help co-users in this setting. We propose a Stage I treatment development study for co-users of marijuana and tobacco who are seeking treatment for tobacco addiction. We plan to adapt an existing evidence-based brief intervention for marijuana users and integrate it into tobacco quitline counseling protocols. The Marijuana Check-up program (MJCU) is an evidence-based, brief behavioral intervention for MJ users not seeking treatment for MJ. The MJCU utilizes motivational enhancement therapy, a form of motivational interviewing (MI), and cognitive behavioral (CBT) strategies to help co-users: 1) understand the health and behavioral consequences of using MJ, 2) consider making changes to their use, and 3) cut down on the frequency, quantity and methods of using MJ. The theoretical constructs (MI, CBT) and the mode of delivery (phone, web, and mail) are similar for both treatments (quitline and MJCU) and thus are potentially a good fit for integration. We will conduct a small randomized pilot study with 100 co-users recruited from four state quitlines to determine the feasibility, acceptability and preliminary effectiveness of the integrated intervention compared with the standard quitline treatment alone. Outcomes will include treatment engagement (calls completed), satisfaction with treatment, tobacco cessation at 3 months (biochemically verified), and marijuana outcomes at 3 months. Our approach uses an opportunistic, real-world setting to offer marijuana counseling services to co-users who may be more willing to consider reducing their marijuana use given their motivation to quit tobacco and the known risks of meeting tobacco cessation goals due to continued use of marijuana. The proposed brief behavioral intervention addressing co-use may increase quitline callers' chances of achieving and maintaining tobacco abstinence and increasing their motivation to reduce marijuana use. Findings will inform development of scalable public health intervention strategies for co-users easily implemented across quitlines.