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We present a new method to estimate the fraction of stars with chemicalcomposition of first and second(s) generation(s) currently hosted in Galacticglobular clusters (GCs). We compare cluster and field stars of similarmetallicity in the [Fe/H]-[Na/H] plane. Since the phenomenon of multiplepopulations is only restricted to the cluster environment, the number of GCstars whose location coincides with that of field stars provides the fractionof first generation stars in that cluster. By exclusion, the fraction of secondgeneration stars is derived. We assembled a dataset of 1891 field stars of thethin disk, thick disk, and halo of the Milky Way in the metallicity range -3.15<= [Fe/H] <= +0.48 dex and with Na abundance from high resolution spectra. Theyare mostly dwarfs, but include also giants. Considering only the range inmetallicity spanned by most GCs extensively studied for the Na-Oanticorrelation (-2.36 <= [Fe/H] <= -0.33 dex), we have 804 stars. The totalsample is homogeneized by offsets in [Fe/H] and [Na/H] with respect to areference sample using the same line list and NLTE correction for Na adopted ina recent extensive survey of GC stars. This fully accounts for offsets amonganalyses due to different temperature scales, line lists, adopted (orneglected) corrections for departures from LTE. We illustrate our methodestimating the fraction of first and second generation stars in the wellstudied GC NGC 6752. As a by-product, the comparison of [Na/H] values in GC andfield stars suggests that at least two classes of old stellar systems probablycontributed to the halo assembly: one group with characteristics similar to thecurrently existing GCs, and the other more similar to the present-day dwarfsatellite galaxies.

The fraction of first and second generation stars in globular clusters. I The case of NGC 6752