  Starting with the well-known Nambu-Goto action for an N-extended body system
the propagator in the microcanonical ensemble is explicitly computed. This
propagator is independent of the temperature and, in contrast with the previous
references, takes account on all the non-local effects produced by the extended
objects (e.g., strings) in interaction. The relation between relativistic
quantum field theories in the microcanonical approach and the pure S-matrix
formulation is stablished and analyzed.
