  We present an analysis of the 2-point correlation function, of the X-ray
Brightest Abell-type Cluster sample (XBACs; Ebeling et al. 1998) and of the
cosmological constraints that it provides. If \xi(r) is modelled as a
power-law, we find r_0=26.0 +/- 4.5 Mpc/h and \gamma=2.0 +/-0.4, with errors
corresponding to 2\sigma uncertainties. Only a marginal increase of the
correlation amplitude is found as the flux limit is increased from 5x10^{-12}
cgs to 12x10^{-12} cgs, thus indicating a weak dependence of the correlation
amplitude on the cluster X-ray luminosity. Furthermore, we present a method to
predict correlation functions for flux-limited X-ray cluster samples from
cosmological models. The method is based on the analytical recipe by Mo & White
(1996) and on an empirical approach to convert cluster fluxes into masses. We
use a maximum-likelihood method to place constraints on the model parameter
space. We find that the shape parameter of the power spectrum lies in the
2\sigma range 0.05<\Gamma<0.20. As for the amplitude of the power-spectrum, we
find \sigma_8 in the range 0.4-0.8 for \Omega_0=1 and 0.8-2.0 for \Omega_0=0.3.
This result is in agreement with, although less constraining than, results
based on the local cluster abundance.
