  We present X-ray observations of two high redshift clusters of galaxies
carried out with the BeppoSAX satellite. One cluster, RXJ0152.7-1357 at z =
0.83, was selected from the ROSAT Deep Cluster Survey sample, as one of the
most X-ray luminous systems known at z>0.5. The optical and ROSAT-PSPC data
show a complex morphology with at least two cores. Our SAX observations yield a
gas temperature kT = 6.46^{+1.74}_{-1.19} keV and a metallicity A =
0.53^{+0.29}_{-0.24}, with a prominent iron K(alpha) line. The second cluster,
MS2053.7-0449 at z = 0.58, was selected from the EMSS sample. Given the poor
statistics no constraints on the metallicity can be derived from the present
data. Large uncertainties are associated to the gas temperature (kT =
6.7^{+6.8}_{-2.3} keV), which has been obtained after fixing the abundance to
0.3 solar. Combining these results with those obtained for similarly high
redshift (z >= 0.5) clusters of galaxies with broad band X-ray spectra, we
discuss the high redshift L(bol) - T relationship. The data can easily be
accommodated with a non-evolving (or mildly evolving) L(bol) - T relation, a
result which, when combined with the little observed evolution in the bulk of
the X-ray cluster population, gives support to low Omega cosmological models.
