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.