This article describes the simultaneous determination of bedaquiline fumarate (TMC-207) and rifabutin in human plasma by stable isotope dilution tandem mass spectrometry. The methodology was developed for an investigation of potential drug-drug interactions of the two anti-tuberculosis drugs when given together to healthy human volunteers. Use of the two drugs in combination to treat disease caused by Mycobacterium tuberculosis is contemplated as the bacterium becomes resistant to many currently available drugs.