  The presence of a ``warm absorber'' was first suggested to explain spectral
variability in an X-ray spectrum of the radio-quiet QSO MR2251-178. A unified
picture, in which X-ray warm absorbers and ``intrinsic'' UV absorbers are the
same, offers the opportunity to probe the nuclear environment of active
galactic nuclei. To test this scenario and understand the physical properties
of the absorber, we obtained a UV spectrum of MR2251-178 with the Faint Object
Spectrograph (FOS) onboard the Hubble Space Telescope (HST). The HST spectrum
clearly shows absorption due to Ly-alpha, NV and CIV, blueshifted by 300 km/sec
from the emission redshift of the QSO. The rarity of both X-ray and UV
absorbers in radio-quiet QSOs suggests these absorbers are physically related,
if not identical. Assuming the unified scenario, we place constraints on the
physical parameters of the absorber and conclude the mass outflow rate is
essentially the same as the accretion rate in MR2251-178.
