  We test the effects of magnetic fields on the mass determination of galaxy
clusters with the X-ray method. As this method takes into account only thermal
pressure, additional non-thermal pressure support, e.g. caused by a magnetic
field, can result in an underestimation of the mass. SPH models of galaxy
clusters including magnetic fields of various strengths in two different
cosmologies are used for the test. We compare the true mass of the model
clusters with the mass determined from simulated X-ray images of the same
models. The effect of the magnetic field on the mass in relaxed clusters is
found to be negligible compared to other uncertainties, even at small radii,
where the strongest effects are expected. However in one model, in which the
cluster is in the process of merging, we find an effect of up to a factor of 2
caused by the magnetic field. In relaxed clusters the mass estimate, averaged
over many models, is at most 15% smaller in models with relatively high
magnetic field of $2\mu$G compared to models without a magnetic field with a
small dependence on the cosmology chosen. Therefore we conclude that the
presence of a magnetic field cannot be responsible for the discrepancies of a
factor of 2-3 found between X-ray and lensing mass in the central regions of
relaxed clusters.
