  A method is presented here for investigating variations in the upper end of
the stellar Initial Mass Function (IMF) by probing the production rate of
ionizing photons in unresolved, compact star clusters with ages<10 Myr and
covering a range of masses. We test this method on the young cluster population
in the nearby galaxy M51a, for which multi-wavelength observations from the
Hubble Space Telescope are available. Our results indicate that the proposed
method can probe the upper end of the IMF in galaxies located out to at least
10 Mpc, i.e., a factor 200 further away than possible by counting individual
stars in young compact clusters. Our results for this galaxy show no obvious
dependence of the upper mass end of the IMF on the mass of the star cluster,
down to ~1000 M_sun, although more extensive analyses involving lower mass
clusters and other galaxies are needed to confirm this conclusion.
