Using the theoretical results of Barbuy, the variation of the Mg_2 index as a function of the stellar parameters (T_eff, logg, [Fe/H]), including the Mg/Fe ratio, is obtained. This allows a new calibration of the metallicity indicators Mg_2 and [Fe] through synthetic indices computed for stellar populations having a metallicity spread and ages of the order of 14-16 Gyr. The models indicate that the integrated Mg_2 index depends on the chemical history, since it varies not only with the metallicity but also with the average Mg/Fe ratio. The study of a sample of elliptical galaxies shows that on average, [Mg/Fe]=0.40, comparable to the values observed in metal-poor galactic halo stars. This confirms the early suspicions that elliptical galaxies (Es) had a very high star formation rate in the past, and that the enrichment was caused by massive stars.