  A sufficient integral criterion for a blow-up solution of the Hopf equations
(the Euler equations with zero pressure) is found. This criterion shows that a
certain positive integral quantity blows up in a finite time under specific
initial conditions. Blow-up of this quantity means that solution of the Hopf
equation in 3D can not be continued in the Sobolev space $H^2({\cal R}^3)$ for
infinite time.
