  The influence of nonperturbative fields on instantons in quantum
chromodynamics is studied. Effective action for instanton is derived in bilocal
approximation and it is demonstrated that stochastic background gluon fields
are responsible for IR stabilization of instantons. It is shown that instanton
size in QCD is of order of 0.25 fm. Comparison of obtained instanton size
distribution with lattice data is made.
