  From an up-to-date compilation of ACO cluster redshifts and velocity
dispersions we extract a homogeneous sample of 459 clusters with robust
velocity dispersion. Using the virial theorem to estimate cluster masses, and a
correlation between Abell galaxy counts and R-band luminosity, we find a median
M/L_R of 174 h_50 M_sun/L_sun. However, if we correct the virial masses
according to the X-ray vs. virial mass correlation, as derived from our
analysis, we obtain M/L_R ~100 h_50 M_sun/L_sun. These two values of M/L imply
a mass content of the Universe of Omega_m ~0.26 and 0.15, respectively.
