  Using the brightest clusters in the ROSAT All-Sky Survey we compiled an X-ray
flux-limited sample of galaxy clusters. The clusters have been reanalysed using
ROSAT PSPC pointed observations if possible. The gravitational mass has been
determined individually for each cluster in a homogeneous way assuming
hydrostatic equilibrium. The mass function has been derived. We present the
preliminary results and a comparison to previous determinations.
