  We report the results of a multi-wavelength campaign on the blazar Mrk 421
during outburst. We observed four strong flares at X-ray energies that were not
seen at other wavelengths (partially because of missing data). From the fastest
rise in the X-rays, an upper limit could be derived on the extension of the
emission region. A time lag between high-energy and low-energy X-rays was
observed, which allowed an estimation of the magnetic-field strength. The
spectral analysis of the X-rays revealed a slight spectral hardening of the
low-energy (3 - 43 keV) spectral index. The hardness-ratio analysis of the
Swift-XRT (0.2 - 10 keV) data indicated a small correlation with the intensity;
i. e., a hard-to-soft evolution was observed. At the energies of IBIS/ISGRI (20
- 150 keV), such correlations are less obvious. A multiwavelength spectrum was
composed and the X-ray and bolometric luminosities are calculated.
