  Observations of AGNs and microquasars by ASCA, RXTE, Chandra and XMM-Newton
indicate the existence of wide X-ray emission lines of heavy ionized elements
in their spectra. The emission can arise in the inner parts of accretion discs
where the effects of General Relativity (GR) must be counted, moreover such
effects can dominate. We describe a procedure to estimate an upper limit of the
magnetic fields in the regions where X-ray photons are emitted. We simulate
typical profiles of the iron $K_\alpha$ line in the presence of a magnetic
field and compare them with observational data. As an illustration we find $H <
10^{10} - 10^{11}$ Gs for Seyfert galaxy MCG--6--30--15. Using the perspective
facilities of measurement devices (e.g. Constellation-X mission) a better
resolution of the blue peak structure of iron $K_\alpha$ line will allow to
find the value of the magnetic fields if the latter are high enough.
