  We follow the growth of baryonic structure in the presence of a magnetic
field within an approximate cosmological magneto-hydrodynamic simulation,
produced by adding an (isotropic) magnetic pressure related to the local gas
pressure. We perform an ensemble of these simulations to follow the
amplification of the field with time. By using a variety of initial field
strengths and changing the slope of the power law that governs the way the
field grows with increasing density we span the range of current observations
and demonstrate the size of the effect realistic magnetic fields could have on
the central density of groups and clusters. A strong magnetic field
significantly reduces the central gas density which, in turn, reduces
observable quantities such as the X-ray luminosity.
