  We describe the relationship between time-ordered and retarded response
functions in a plasma. We obtain an expression, including the proper
$i\epsilon$-prescription, for the induced current due to hard thermal loops in
a non-Abelian theory, thus giving the non-Abelian generalization of the Kubo
formula. The result is closely related to the eikonal for a Chern-Simons theory
and is relevant for a gauge-invariant description of Landau damping in the
quark-gluon plasma at high temperature.
