The peculiar emission line star MWC349 hosts the only known hydrogen
recombination line maser known to date. We propose to investigate
this unique maser whose line flux is found to increase steeply up
to the shortest wavelength accessible from the ground, at ISO wave-
lengths.The principal questions addressed are: (i) what is the form of
growth of maser line flux with wavelength, and (ii) at what wavelength
does the maser finally cut off.

We expect the saturated growth which characterizes the submm maser
transitions to terminate somewhere in the range 300 to 30 microns either
due to the maser running out of gain or to atomic processes which
quench the level inversion. Measurements of the form of the cutoff and
of the source continuum will help to decide between these mechanisms.
The form of growth of the maser line flux gives information on the
length of the gain path as a function of wavelength. These data help
to decide between possible locations of the maser on the disk, either
upper and lower surface, or its inner edge. Interpretation of
maser velocities will then be possible in terms of disk kinematics and
the mass of the system. In this way, the study of the maser with ISO
will help to clarify the nature of the disc around this enigmatic
source which is most consistently interpreted as a very massive ZAMS
star which has dispersed its parental cloud, but which is still
surrounded by its accretion disk.