  We explore the possibility to calibrate massive cluster ellipticals as
cosmological standard rods. The method is based on the Fundamental Plane
relation combined with a correction for luminosity evolution which is derived
from the Mg$-\sigma$ relation. Principle caveats and sources of major errors
are briefly discussed.
  We apply the described procedure to nine elliptical galaxies in two clusters
at $z=0.375$ and derive constraints on the cosmological model. For the best
fitting $\Lambda$-free cosmological model we obtain: $q_o \approx 0.1$, with
90% confidence limits being $0 < q_o < 0.7$ (the lower limit being due to the
presence of matter in the Universe). If the inflationary scenario applies
(i.e., space has flat geometry), then, for the best fitting model, matter and
$\Lambda$ contribute about equally to the critical cosmic density (i.e.
$\Omega_m \approx \Omega_\Lambda \approx 0.5$). With 90% confidence
$\Omega_\Lambda$ should be smaller than 0.9.

Astrophysics