  We consider a contribution of microlensing to the X-ray variability of
high-redshifted QSOs. Such an effect could be caused by stellar mass objects
(SMO) located in a bulge or/and in a halo of this quasar as well as at
cosmological distances between an observer and a quasar. Cosmologically
distributed gravitational microlenses could be localized in galaxies (or even
in bulge or halo of gravitational macrolenses) or could be distributed in a
uniform way. We have analyzed both cases of such distributions. As a result of
our analysis, we obtained that the optical depth for microlensing caused by
stellar mass objects is usually small for quasar bulge and quasar halo
gravitational microlens distributions. On the other hand, the optical depth for
gravitational microlensing caused by cosmologically distributed deflectors
could be significant and could reach 0.01 - 0.1 at z ~ 2. This means that
cosmologically distributed deflectors may contribute significantlly to the
X-ray variability of high-redshifted QSOs (z > 2). Considering that the upper
limit of the optical depth (~ 0.1) corresponds to the case where dark matter
forms cosmologically distributed deflectors, observations of the X-ray
variations of unlensed QSOs can be used for the estimation of the dark matter
fraction of microlenses.
