  We present for the first time a high-resolution X-ray spectrum of a Seyfert
galaxy. The Chandra-LETGS spectrum of NGC 5548 shows strong, narrow absorption
lines from highly ionised species (the H-like and He-like ions of C, N, O, Ne,
Na, Mg, Si, as well as Fe XIV--XXI). The lines are blueshifted by a few hundred
km/s. The corresponding continuum absorption edges are weak or absent. The
absorbing medium can be modelled by an outflowing, thin and warm shell in
photoionization equilibrium. The absorption lines are similar to lower
ionization absorption lines observed in the UV, although these UV lines
originate from a different location or phase of the absorbing medium.
Redshifted with respect to the absorption lines, emission from the O VIII Lyman
alpha line as well as the O VII triplet is visible. The flux of these lines is
consistent with emission from the absorbing medium. The O VII triplet intensity
ratios demonstrate that photoionization dominates and yield an upper limit to
the electron density.
