  The vacuum expectation value of the stress-energy tensor is calculated for
spin $1\over 2$ massive fields in several multiply connected flat spacetimes.
We examine the physical effects of topology on manifolds such as $R^3 \times
S^1$, $R^2\times T^2$, $R^1 \times T^3$, the Mobius strip and the Klein bottle.
We find that the spinor vacuum stress tensor has the opposite sign to, and
twice the magnitude of, the scalar tensor in orientable manifolds. Extending
the above considerations to the case of Misner spacetime, we calculate the
vacuum expectation value of spinor stress-energy tensor in this space and
discuss its implications for the chronology protection conjecture.
