  We present the K-band (2.2 microns) luminosity functions of the X-ray
luminous clusters MS1054-0321 (z=0.82), MS0451-0305 (z=0.55), Abell 963
(z=0.206), Abell 665 (z=0.182) and Abell 1795 (z=0.063) down to absolute
magnitudes M_K = -20. Our measurements probe fainte absolute magnitudes than do
any previous studies of the near-infrared luminosity function of clusters. All
clusters are found to have similar luminosity functions within the errors, when
the galaxy populations are evolved to redshift $z=0$. It iw known that the most
massive bound systems in the Universe at all redshifts are X-ray luminous
clusters. Therefore, assuming that the clusters in our sample correspond to a
single population seen at different redshifts, the results here imply that not
only had the stars in present-day ellipticals in rich clusters formed by z=0.8,
but that they existed in as luminous galaxies then as they do today.
  Additionally, the clusters have K-band luminosity functions which appear to
be consistent with the Kband field luminosity function in the range -24 < M_K <
-22, although the uncertainties in both the field and cluster samples are
large.
