Context :

Aims : We present a new homogeneous set of metallicity estimates based on Lick indices for the old globular clusters of the M31 galaxy .
The final aim is to add homogeneous spectroscopic metallicities to as many entries as possible of the Revised Bologna Catalog of M31 clusters RBC Version 4 available at : www.bo.astro.it/M31 , by reporting Lick indices measurements from any source ( literature , new observations , etc . )
into the same scale .

Methods : New empirical relations of [ Fe/H ] as a function of [ MgFe ] and Mg2 indices , as defined by Trager et al .
( 1998 ) , are based on well studied Galactic Globular Clusters , complemented with theoretical model predictions for -0.2 \leq [ Fe / H ] \leq + 0.5 .
Lick indices for M31 clusters from various literature sources ( 225 clusters ) and from new observations by our team ( 71 clusters ) have been transformed into the Trager et al .
( 1998 ) system , yielding new metallicity estimates for 245 globular clusters of M31 .

Results : Our values are in good agreement with recent estimates based on detailed spectral fitting and with those obtained from Color Magnitude Diagrams of clusters imaged with the Hubble Space Telescope .
The typical uncertainty on individual estimates is \simeq \pm 0.25 dex , as resulted from the comparison with metallicities derived from Color Magnitude Diagrams of individual clusters .

Conclusions : The metallicity distribution of M31 globular cluster is briefly discussed and compared with that of the Milky Way .
Simple parametric statistical tests suggest that the distribution is likely not unimodal .
The strong correlation between metallicity and kinematics found in previous studies is confirmed .
The most metal-rich GCs tend to be packed at the center of the system and to cluster tightly around the galactic rotation curve defined by the HI disk , while the velocity dispersion about the curve increases with decreasing metallicity .
However , also the clusters with [ Fe / H ] < -1.0 display a clear rotation pattern , at odds with their Milky Way counterparts .