We provide a new calibration of the metallicity dependence of the Mg2 index in stellar systems. It is based on new isochrones by the Padova group and the latest stellar spectral libraries. Since the Mg2 line is one of the strongest absorption lines in the optical spectrum, it is often used to determine metallicities of composite stellar systems assuming a certain age. This paper allows one to do this, with an uncertainty of about 0.1 in log Z, for an age range between 8 and 17 Gyr. We estimate that the uncertainties of the current Mg2 calibrations in the literature amount to another 0.15 in log Z. Using our calibration we find, under conservative assumptions, that the large majority of elliptical galaxies, and also many bulges of spiral galaxies, must have Mg abundances larger than solar; this conclusion depends very little on their star formation history. If [Mg/ Fe] = 0, the same can be said for total metallicities. If we assume that [Mg/Fe] = 0.45, only about 38% of the ellipticals must have metallicities (Z) larger than solar.