  Past X-ray observations by ASCA suggest that warm absorbers (O VII and O VIII
edges) are apparently rare in high luminosity AGNs (quasars) while they are
more common in low luminosity AGNs (Seyferts). However, this could be a
selection effect if high luminosity AGNs have mostly narrow absorption lines
(with no strong bound free edges), which escaped detection by the low
resolution of ASCA. To check this hypothesis we are studying the
high-resolution X-ray spectra of quasars from grating spectrometers on board
Chandra and XMM-Newton in search for absorption lines. In this contribution we
present spectra of three quasars. The spectra show narrow (several hundred
km/s) absorption and emission X-ray lines from H-like and He-like ions of O,
Ne, Mg, and other abundant elements. We also detect absorption from iron
L-shell lines and iron M-shell unresolved transition array. We present the
analysis of MR2251-178 where we find that at least two, and probably three,
distinct warm absorbers are needed to explain the high resolution spectrum of
this object. We re-analyze the high-resolution X-ray spectrum of PG1211+143 and
suggest that an outflow velocity of about 3000 km/s provides an adequate
explanation to these data. We also present preliminary results form the
Chandra/HETGS observation of the quasar 4C74.26.
