The hydroxyl radical (OH) is the primary oxidant in the atmosphere, responsible for many photochemical reactions that affect both regional air quality and global climate change. Because of its high reactivity, abundances of OH in the troposphere are less than 1 part per trillion by volume (pptv) and thus difficult to measure accurately. This paper describes an instrument for the sensitive detection of OH in the troposphere using low-pressure laser-induced fluorescence. Ambient air is expanded into a low-pressure detection chamber, and OH is both excited and detected using the A2&Sgr;+(v'=0)→X2&Pgr;(v'=0) transition near 308 nm. An injector upstream of the detection axis allows for the addition of reagent NO to convert ambient HO2 to OH using the fast reaction HO2+NO→OH+NO2. Using recent advances in laser and detector technologies, this prototype instrument is able to detect less than 1¿105 molecules cm-3 (0.004 pptv) of OH with an integration time of 30 s with negligible interferences.