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We present a new approach in the study of the Initial Mass function (IMF) inexternal galaxies based on quasar microlensing observations. We usemeasurements of quasar microlensing magnifications in 24 lensed quasars toestimate the average mass of the stellar population in the lens galaxieswithout any a priori assumption on the shape of the IMF. The estimated meanmass of the stars is $\langle M \rangle =0.16^{+0.05}_{-0.08} M_\odot$ (at 68\%confidence level). We use this average mass to put constraints into twoimportant parameters characterizing the IMF of lens galaxies: the low-massslope, $\alpha_2$, and the low-mass cutoff, $M_{low}$. Combining theseconstraints with prior information based on lensing, stellar dynamics, andabsorption spectral feature analysis, we calculate the posterior probabilitydistribution for the parameters $M_{low}$ and $\alpha_2$. We estimate valuesfor the low-mass end slope of the IMF $\langle \alpha_2\rangle=-2.6\pm 0.9$(heavier than that of the Milky Way) and for the low-mass cutoff $\langleM_{low}\rangle=0.13\pm0.07$. These results are in good agreement with previousstudies on these parameters and remain stable against the choice of differentsuitable priors.

The Initial Mass Function of Lens Galaxies from Quasar Microlensing