  The property of gluon Reggeization plays an essential role in the derivation
of the Balitsky-Fadin-Kuraev-Lipatov (BFKL) equation for the cross sections at
high energy $\sqrt s$ in perturbative QCD. This property has been proved to all
orders of perturbation theory in the leading logarithmic approximation and it
is assumed to be valid also in the next-to-leading logarithmic approximation,
where it has been checked only to the first three orders of perturbation
theory. From $s$-channel unitarity, however, very stringent ``bootstrap''
conditions can be derived which, if fulfilled, leave no doubts that gluon
Reggeization holds.
