  We apply a new bosonization technique to relativistic field theories of
fermions whose partition function is dominated by bosonic composites, and
derive the effective action for these bosons. The derivation respects all
symmetries, including gauge invariance, with the exception of Euclidean
invariance which must be checked a posteriori. We use a lattice regularization
which should make applications to gauge theories easier. We test the method on
a fermion field theory with quartic interaction in the limit when the number of
flavours N_f is large, and show that it reproduces the exact results in the
bosonic sector, namely condensation of a compositeboson with the right mass
which breaks the discrete chiral invariance of the model. Moreover we determine
the structure function of the condensed composite, whose spatial part turns out
to be identical to that of the Cooper pairs of the BCS model of
superconductivity.
