  For any non-zero finite module M of finite projective dimension over a
noetherian local ring R with maximal ideal m and residue field k, it is proved
that the natural map Ext_R(k,M)-->Ext_R(k,M/mM) is non-zero when R is regular
and is zero otherwise. A noteworthy aspect of the proof is the use of stable
cohomology. Applications include computations of Bass series over certain local
rings.
