Abstract
The classes of extragalactic objects identified with X-ray sources are: clusters of galaxies, Seyfert galaxies, other types of (active) galaxies, QSO’s, BL Lac objects and, possibly, superclusters. For a review of the properties of extragalactic X-ray sources one is referred to Gursky and Schwarz (1977). The relative occurrence of the different classes of objects among the high-latitude (|b| > 20°) X-ray sources in the 4U (Forman et al. 1978) and 2A (Cooke et al. 1978) catalogues is given in Table 1. The results from the two catalogues are not at all independent, because they have a large number of sources in common. The higher identification rate in the 2A catalogue reflects the generally smaller error boxes in this catalogue, compared with those in the 4U catalogue.
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