Theoretical calculations of OH energy level populations under
 conditions relevant to star-forming regions often yield
 population inversions in transitions in the FIR in addition to
 the well known lambda doublet inversions which lead to observed
 OH maser action. Some OH FIR transitions have been observed but
 none have been observed to mase. The transitions which may be
 inverted according to theory, however, have not been studied.
   Lines which may be inverted are at 115.14, 134.83 and 135.95
 microns and are all cross-stack (F2->F1) transitions in OH. Only
 the two longer wavelength lines are likely to be observable with
 ISO. The main goal of the proposal is to detect for the first
 time FIR laser emission from space with the LWS/Fabry-Perot
 system in AOT LWS04 mode. ISO is the only instrument presently
 available which is capable of detecting these lines.