  NGC 6388 is one of the most massive Galactic globular clusters (GC) and it is
an old, metal-rich, Galactic bulge cluster. By exploiting previous
spectroscopic observations we were able to bypass the uncertainties in
membership related to the strong field stars contamination. We present the
abundance analysis of 12 new giant stars with UVES spectra and 150 giants with
GIRAFFE spectra acquired at the ESO-VLT. We derived radial velocities,
atmospheric parameters and iron abundances for all stars. When combined to
previous data, we obtain a grand total of 185 stars homogeneously analysed in
NGC 6388 from high-resolution spectroscopy. The average radial velocity of the
185 stars is 81.2+/-0.7, rms = 9.4 km/s. We obtain an average metallicity
[Fe/H]=-0.480 dex, rms = 0.045 dex (35 stars) and [Fe/H]=-0.488 dex, rms =
0.040 dex (150 stars) from the UVES and GIRAFFE samples, respectively.
Comparing these values to internal errors in abundance, we exclude the presence
of a significant intrinsic metallicity spread within the cluster. Since about a
third of giants in NGC 6388 is claimed to belong to the "anomalous red giants"
in the HST pseudo-colour map defining the so-called type-II GCs, we conclude
that either enhanced metallicity is not a necessary requisite to explain this
classification (as also suggested by the null iron spread for NGC 362) or NGC
6388 is not a type-II globular cluster.
