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 } \simeq 174 h _ { 50 } M _ { \odot } / L _ { \odot } .
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 } \simeq 100 h _ { 50 } M _ { \odot } / L _ { \odot } . These two values of M / L imply a mass content of the Universe of \Omega _ { m } \simeq 0.26 and 0.15 , respectively .