  In this paper we report that a bright, X-ray source in the Antennae galaxies
(NGC 4038/9), previously identified as an ultra-luminous X-ray source, is in
fact a background quasar. We identify an isolated infrared and optical
counterpart within 0.3+/-0.5 arcseconds the X-ray source X-37. After acquiring
an optical spectrum of its counterpart, we use the narrow [OIII] and broad
H_alpha emission lines to identify X-37 as a quasar at a redshift of z=0.26.
Through a U, V, and K_s photometric analysis, we demonstrate that most of the
observable light along this line of sight is from the quasar. We discuss the
implications of this discovery and the importance of acquiring spectra for
optical and IR counterparts to ULXs.
