  Let $R$ be a (commutative Noetherian) local ring of prime characteristic that
is $F$-pure. This paper studies a certain finite set ${\mathcal I}$ of radical
ideals of $R$ that is naturally defined by the injective envelope of the simple
$R$-module. This set ${\mathcal I}$ contains $0$ and $R$, and is closed under
taking primary components. For a multiplicatively closed subset $S$ of $R$, the
concept of tight closure with respect to $S$, or $S$-tight closure, is
discussed, together with associated concepts of $S$-test element and $S$-test
ideal. It is shown that an ideal of $R$ belongs to ${\mathcal I}$ if and only
if it is the $S'$-test ideal of $R$ for some multiplicatively closed subset
$S'$ of $R$. When $R$ is complete, ${\mathcal I}$ is also `closed under taking
test ideals', in the following sense: for each proper ideal $C$ in ${\mathcal
I}$, it turns out that $R/C$ is again $F$-pure, and if $J$ and $K$ are the
unique ideals of $R$ that contain $C$ and are such that $J/C$ is the (tight
closure) test ideal of $R/C$ and $K/C$ is the big test ideal of $R/C$, then
both $J$ and $K$ belong to ${\mathcal I}$. The paper ends with several
examples.
