  The S-matrix approach is applied to charged and neutral pion production in pp
collisions, in which the spin/isospin channels filter differently the
rescattering mechanism. The chiral perturbation theory rescattering amplitude
is employed. The contributions from the single-nucleon, Z-diagrams and
Delta-isobar mechanisms are also included. For the reactions investigated, the
convergence of the partial wave series is quite satisfactory and J=0 is enough
to describe the data close to threshold.
