  It is shown that two reacting cosmological fluids, each of them perfect on
its own, which exchange energy and momentum without preserving particle
numbers, give rise to an entropy producing `reactive' bulk stress of the system
as a whole, as soon as the detailed balance between decay and inverse decay
processes is perturbed. This demonstrates explicitly that particle generation
is dynamically equivalent to an effective bulk pressure. We derive a
semiquantitative formula for the corresponding new kinetic coefficient and
evaluate the latter for the out-of-equilibrium decay of heavy, nonrelativistic
particles into radiation. It turns out that the associated reactive bulk
viscosity may be more than one order of magnitude larger than the conventional
bulk viscosity, calculated, e.g., in radiative hydrodynamics.
