We consider open
3-manifolds that are monotone unions of compact 3-manifolds each bounded by a
torus. We give necessary and sufficient conditions for embedding such an
open 3-manifold in a compact 3-manifold. We also show that if the open
3-manifold embeds in a compact 3-manifold, then it embeds in a compact
3-manifold as the complement of the intersection of a decreasing sequence of solid
tori.