  Let $R$ be an associative ring with unit. This paper deals with various
aspects of the category of functors of $\mathcal R$-modules; that is, the
category of additive and covariant functors from the category of R-modules to
the category of abelian groups. We give several characterizations of left-exact
Mittag-Leffler functors of $\mathcal R$-modules.
