  Let R and S be differential graded algebras. In this paper we give a
characterisation of when a differential graded R-S-bimodule M induces a full
embedding of derived categories M\otimes - :D(S)--> D(R). In particular, this
characterisation generalises the theory of Geigle and Lenzing's homological
epimorphisms of rings. Furthermore, there is an application of the main result
to Dwyer and Greenlees's Morita theory.
