  The simplest interpretation of the microlensing events observed towards the
Large Magellanic Cloud is that approximately half of the mass of the Milky Way
halo is in the form of MAssive Compact Halo Objects with $M \sim 0.5
M_{\odot}$. This poses severe problems for stellar MACHO candidates, and leads
to the consideration of more exotic objects such as primordial black holes
(PBHs). Constraining the MACHO mass function will shed light on their nature.
Using the current data we find, for four halo models, the best fit
delta-function, power law and PBH mass functions. The best fit PBH mass
functions, despite having significant finite width, have likelihoods which are
similar to, and for one particular halo model greater than, those of the best
fit delta functions. We also find that if the correct halo model is known then
$\sim$ 500 events will be sufficient to determine whether the MACHO mass
function has significant width, and will also allow determination of the mass
function parameters to $\sim 5%$.
