On abelian groups with commuting monomorphisms

We find necessary and sufficient conditions under which an abelian group with commuting monomorphisms has the endomorphism ring normal. We describe divisible groups with left- or righ-tinvariant monomorphisms and prove that a reduced torsion-free group with left- or right-invariant monomorphisms has the endomorphism ring normal. Necessary and sufficient conditions are indicated for the left and right invariance of monomorphisms from some classes of splitting groups.