  We study the clustering of the mass distribution in COBE-normalized open and
flat CDM models using large N-body simulations. With an age of the universe of
14 Gyr (12 Gyr) for the flat (open) models and a baryon density fixed by
nucleosynthesis constraints, the observed abundance of rich galaxy clusters
leads to tight constraints on the density parameter; 0.25<Omega_0<0.4
(0.4<Omega_0<0.5) for flat (open) models. The upper limits on Omega_0 can be
relaxed if one lowers the H_0 and increases the age of the universe, but h<0.25
is required to allow Omega_0=1. The constraints also change if one allows
tilted primordial power spectra. An acceptable Omega_0=1 model with h=0.5 can
be constructed, with a tilt of n=0.8. We compare the evolved mass correlation
functions of the most promising models with those of APM galaxies. The flat
models have steep correlation functions at small scales and require galaxies to
be antibiased on scales r<8 Mpc/h. The open models require little or no
antibias on small scales and a positive bias on large scales. The lack of a
positive bias on small scales is difficult to reconcile with the virial
analysis of galaxy clusters which, if Omega_0>0.2, implies that galaxies are
overabundant in clusters relative to the field. The tilted Omega_0=1 model, on
the other hand, does require that galaxies be positively biased on all scales.
We also compute the topology of isodensity contours in these models, obtaining
theoretical predictions that are less sensitive to galaxy bias.
