  We consider a contribution of microlensing to the X-ray variability of
high-redshifted QSOs. 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. We found that the optical depth for gravitational
microlensing caused by cosmologically distributed deflectors could be
significant and could reach $10^{-2} - 0.1$ at $z\sim 2$. 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 ($\tau\sim 0.1$) 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.
