  We evaluate in a homogeneous way the optical masses of 170 nearby clusters
(z< 0.15). The sample includes both data from the literature and the new ENACS
data (Katgert et al. 1996, 1998). On the assumption that mass follows the
galaxy distribution, we compute the masses of each cluster by applying the
virial theorem to the member galaxies. We constrain the masses of very
substructured clusters (about 10% of our clusters) between two limiting values.
After appropriate rescaling to the X-ray radii, we compare our optical mass
estimates to those derived from X-ray analyses, which we compiled from the
literature (for 66 clusters). We find a good overall agreement. This agreement
is expected in the framework of two common assumptions: that mass follows the
galaxy distribution, and that clusters are not far from a situation of
dynamical equilibrium with both gas and galaxies reflecting the same underlying
mass distribution. We stress that our study strongly supports the reliability
of present cluster mass estimates derived from X-ray analyses and/or
(appropriate) optical analyses.
