  We show, through Monte Carlo simulations, that the Alcock-Paczynski test, as
applied to quasar clustering, is a powerful tool to probe the cosmological
density and equation of state parameters, Omega_{m0}, Omega_{x0} and w. By
taking into account the effect of peculiar velocities upon the correlation
function we obtain, for the Two-Degree Field QSO Redshift Survey (2QZ), the
predicted confidence contours for the cosmological constant (w = -1) and
spatially flat (Omega_{m0} + Omega_{x0}=1) cases. It turns out that, for w =
-1, the test is especially sensitive to the difference Omega_{m0} -
Omega_{Lambda 0}, thus being ideal to combine with CMB results. We also find
out that, for the flat case, it is competitive with future supernova and galaxy
number count tests, besides being complementary to them.
