  We present a theoretical study on the nature of the ultra-luminous X-ray
source NGC 1313 X-2. We evolved a set of binaries with high mass donor stars
orbiting a 20 M_Sun or a 50-100 M_Sun black hole. Using constraints from
optical observations we restricted the candidate binary system for NGC 1313 X-2
to be either a 50-100 M_Sun black hole accreting from a 12-15 M_Sun main
sequence star or a ~20 M_Sun black hole with a 12-15 M_Sun giant donor. If the
modulation of ~6.12 days recently identified as the orbital period of the
system is confirmed, a ~20 M_Sun black hole model becomes unlikely and we are
left with the only possibility that the compact accretor in NGC 1313 X-2 is a
massive black hole of ~50-100 M_Sun.
