  We present a study of the QCD interactions which do not conserve the
chirality of quarks. These non-perturbative forces are responsible for the
violation of the U_A(1) charge conservation and for the breaking of chiral
symmetry. From a systematic analysis we argue that the leading dynamical
sources chirality flips are the interactions mediated by topological vacuum
field fluctuations. We study in detail the contribution of instantons and
derive a simple model-independent semi-classical prediction. This result can be
used to check on the lattice if instantons are the dynamical mechanism
responsible both for chiral symmetry breaking and for the anomalous violation
of the U_A(1) charge conservation.
