  We test the reliability of mass determination in clusters of galaxies by
X-ray observations. The true mass in cluster models is compared to the mass
derived by the X-ray emission and X-ray temperature of a model assuming
hydrostatic equilibrium and spherical symmetry. In general we find good
agreement between the X-ray mass and the true mass without any systematic over-
or under- estimation. The average deviation within the central 2Mpc has a
r.m.s. of 15%.
