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: .We calculate the degree of foreground/background contaminants among the ULX sample candidates in a published catalogue and compare these computations with available spectroscopic 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/arcsec2 isophote from RC3 nearby galaxies and with X-ray luminosities L_X[ 2-10 keV] ≥ 1039 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.