  We investigate $HST$/ACS and WFPC2 images at the positions of five candidate
microlensing events from a large survey of variability in M31 (MEGA). Three
closely match unresolved sources, and two produce only flux upper limits. All
are confined to regions of the color-magnitude diagram where stellar
variability is unlikely to be easily confused with microlensing. Red variable
stars cannot explain these events (although background supernova are possible
for two). If these lenses arise in M31's halo, they are due to masses $0.15 < m
/ M_\odot < 0.49$ (95% certainty, for a $\delta$-function mass distribution),
brown dwarfs for disk lenses, and stellar masses for bulge lenses.
