  Let $T:A\to X$ be a bounded linear operator, where $A$ is a C$^*$-algebra,
and $X$ denotes an essential Banach $A$-bimodule. We prove that the following
statements are equivalent:
  $(a)$ $T$ is anti-derivable at zero (i.e. $ab =0$ in $A$ implies $T(b) a + b
T(a)=0$);
  $(b)$ There exist an anti-derivation $d:A\to X^{**}$ and an element $\xi \in
X^{**}$ satisfying $\xi a = a \xi,$ $\xi [a,b]=0,$ $T(a b) = b T(a) + T(b) a -
b \xi a,$ and $T(a) = d(a) + \xi a,$ for all $a,b\in A$.
  We also prove a similar equivalence when $X$ is replaced with $A^{**}$. This
provides a complete characterization of those bounded linear maps from $A$ into
$X$ or into $A^{**}$ which are anti-derivable at zero. We also present a
complete characterization of those continuous linear operators which are
$^*$-anti-derivable at zero.
