  We present a new approach in the study of the Initial Mass function (IMF) in
external galaxies based on quasar microlensing observations. We use
measurements of quasar microlensing magnifications in 24 lensed quasars to
estimate the average mass of the stellar population in the lens galaxies
without any a priori assumption on the shape of the IMF. The estimated mean
mass 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 two
important parameters characterizing the IMF of lens galaxies: the low-mass
slope, $\alpha_2$, and the low-mass cutoff, $M_{low}$. Combining these
constraints with prior information based on lensing, stellar dynamics, and
absorption spectral feature analysis, we calculate the posterior probability
distribution for the parameters $M_{low}$ and $\alpha_2$. We estimate values
for 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 $\langle
M_{low}\rangle=0.13\pm0.07$. These results are in good agreement with previous
studies on these parameters and remain stable against the choice of different
suitable priors.
