  (abridged) MR2251-178 was the first quasar initially discovered in X-rays,
and the first one found to host a warm absorber. The quasar turned out to be an
outstanding object in many respects. It has a high ratio of X-ray/optical
luminosity, is surrounded by the largest quasar emission-line nebula known, and
is located in the outskirts of a cluster of galaxies. Here, we present results
from an analysis of the X-ray spectral, temporal, and spatial properties of
this source and its environment based on deep ROSAT observations.
  Remarkably, we do not detect any excess X-ray cold absorption expected to
originate from the giant gas nebula surrounding MR2251-178. This excludes the
presence of a huge HI envelope around the quasar. The X-ray spectrum of
MR2251-178 is best fit by a warm-absorbed powerlaw with an ionization parameter
log U = 0.5 and a column density log N_w = 22.6 which, however, cannot be the
same material as the giant optical emission line nebula. The mean (0.1--2.4)keV
X-ray luminosity amounts to 10^{45} erg/s.
  A spatial analysis shows that the bulk of the X-ray emission from the quasar
is consistent with a point source, as expected in view of the powerlaw-shaped
X-ray spectrum and the rapid X-ray variability we detect. In addition, extended
emission appears at weak emission levels, including a bridge between the quasar
and the cD galaxy of the cluster.
  The X-ray emission from the intra-cluster medium is weak or absent. None of
the other member galaxies of the cluster to which MR 2251-178 belongs, are
detected in X-rays. However, east of the quasar there is a significant excess
of X-ray sources, several of them without optical counterparts on the UK
Schmidt plates.
