  We report our analysis of X-ray data on M101 ULX-1, concentrating on high
state Chandra and XMM-Newton observations. We find that the high state of M101
ULX-1 may have a preferred recurrence timescale. If so, the underlying clock
may have periods around 160 or 190 days, or possibly around 45 days. Its
short-term variations resemble those of X-ray binaries at high accretion rate.
If this analogy is correct, we infer that the accretor is a 20-40 Msun object.
This is consistent with our spectral analysis of the high state spectra of M101
ULX-1, from which we find no evidence for an extreme (> 10^40 ergs/s)
luminosity. We present our interpretation in the framework of a high mass X-ray
binary system consisting of a B supergiant mass donor and a large stellar-mass
black hole.
