We report the detection and spectroscopy of the fundamental vibrational band of the OH radical at 3 mum. The mid-IR radiation was produced by difference frequency generation mixing a Nd-YAG laser and a diode laser (lambda = 0.785 mum). OH detection was demonstrated at high sensitivity by observing a minimum detectable pressure of 0.225 mubar for an optical length of 50 cm. Finally, we identified four OH lines, belonging to two Lambda-doublets, and measured their spectral positions with an accuracy of similar or equal to1.6 x 10(-3) cm(-1). (C) 2003 Elsevier Science B.V. All rights reserved.