  We present results from a narrow-band survey of the field around the high
redshift radio galaxy MRC 2104-242. We have selected Halpha emitters in a
7sq.arcmin field and compared the measured number density with that of a field
sample at similar redshift. We find that MRC 2104-242 lies in an overdensity of
galaxies that is 8.0 +/- 0.8 times the average density of a blank field,
suggesting it resides in a large-scale structure that may eventually collapse
to form a massive cluster. We find that there is more dust obscured star
formation in the protocluster galaxies than in similarly selected control field
galaxies and there is tentative evidence of a higher fraction of starbursting
galaxies in the denser environment. However, on average we do not find a
difference between the star formation rate (SFR)-mass relations of the
protocluster and field galaxies and so conclude that the SFR of these galaxies
at z~2.5 is governed predominantly by galaxy mass and not the host environment.
We also find that the stellar mass distribution of the protocluster galaxies is
skewed towards higher masses and there is a significant lack of galaxies at M <
10^10Msun within our small field of view. Based on the level of overdensity we
expect to find ~22 star forming galaxies below 10^10Msun in the protocluster
and do not detect any. This lack of low mass galaxies affects the level of
overdensity which we detect. If we only consider high mass (M > 10^10.5Msun)
galaxies, the density of the protocluster field increases to ~55 times the
control field density.
