We estimate the fraction of mass that is composed of compact objects in gravitational lens galaxies .
This study is based on microlensing measurements ( obtained from the literature ) of a sample of 29 quasar image pairs seen through 20 lens galaxies .
We determine the baseline for no microlensing magnification between two images from the ratios of emission line fluxes .
Relative to this baseline , the ratio between the continua of the two images gives the difference in microlensing magnification .
The histogram of observed microlensing events peaks close to no magnification and is concentrated below 0.6 magnitudes , although two events of high magnification , \Delta m \sim 1.5 , are also present .
We study the likelihood of the microlensing measurements using frequency distributions obtained from simulated microlensing magnification maps for different values of the fraction of mass in compact objects , \alpha .
The concentration of microlensing measurements close to \Delta m \sim 0 can be explained only by simulations corresponding to very low values of \alpha ( 10 % or less ) .
A maximum likelihood test yields \alpha = 0.05 _ { -0.03 } ^ { +0.09 } ( 90 % confidence interval ) for a quasar continuum source of intrinsic size r _ { s _ { 0 } } \sim 2.6 \cdot 10 ^ { 15 } cm .
This estimate is valid in the 0.1 - 10 M _ { \odot } range of microlens masses .
We study the dependence of the estimate of \alpha with r _ { s _ { 0 } } , and find that \alpha \lesssim 0.1 for r _ { s _ { 0 } } \lesssim 1.3 \cdot 10 ^ { 16 } cm .
High values of \alpha are possible only for source sizes much larger than commonly expected ( r _ { s _ { 0 } } > > 2.6 \cdot 10 ^ { 16 } cm ) .
Regarding the current controversy about Milky Way/LMC and M31 microlensing studies , our work supports the hypothesis of a very low content in MACHOS ( Massive Compact Halo Objects ) .
In fact , according to our study , quasar microlensing probably arises from the normal star populations of lens galaxies and there is no statistical evidence for MACHOS in the dark halos .