  CONTEXT. Observational follow-up programmes for the characterization of
ultra-luminous X-ray sources (ULXs) require the construction of clean samples
of such sources in which the contamination by foreground/background sources is
minimum.
  AIMS. In this article we calculate the degree of foreground/background
contaminants among the ULX sample candidates in the Colbert & Ptak (2002)
catalogue and compare these computations with available spectroscopical
identifications.
  METHODS. We use statistics based on known densities of X-ray sources and
AGN/QSOs selected in the optical. The analysis is done individually for each
parent galaxy. The existing identifications of the optical counterparts are
compiled from the literature.
  RESULTS. More than a half of the ULXs, within twice the distance of the major
axis of the 25 mag/arcsec$^2$ isophote from RC3 nearby galaxies and with X-ray
luminosities $L_X$[2-10 keV] $\ge 10^{39}$ erg/s, are expected to be high
redshift background QSOs. A list of 25 objects (clean sample) confirmed to be
real ULXs or to have a low probability of being contaminant
foreground/background objects is provided.
