  We present the luminosity function (LF) of star clusters in M51 based on
HST/ACS observations taken as part of the Hubble Heritage project. The clusters
are selected based on their size and with the resulting 5990 clusters we
present one of the largest cluster samples of a single galaxy. We find that the
LF can be approximated with a double power-law distribution with a break around
M_V = -8.9. On the bright side the index of the power-law distribution is
steeper (a = 2.75) than on the faint-side (a = 1.93), similar to what was found
earlier for the ``Antennae'' galaxies. The location of the bend, however,
occurs about 1.6 mag fainter in M51. We confront the observed LF with the model
for the evolution of integrated properties of cluster populations of Gieles et
al., which predicts that a truncated cluster initial mass function would result
in a bend in, and a double power-law behaviour of, the integrated LF. The
combination of the large field-of view and the high star cluster formation rate
of M51 make it possible to detect such a bend in the LF. Hence, we conclude
that there exists a fundamental upper limit to the mass of star clusters in
M51. Assuming a power-law cluster initial mass function with exponentional
cut-off of the form NdM ~ M^-b * exp(-M/M_C)dM, we find that M_C = 10^5 M_sun.
A direct comparison with the LF of the ``Antennae'' suggests that there M_C =
4*10^5 M_sun.
