  Due to their short life-time, flavour-neutral mesons cannot be utilized as
free secondary beams or targets, and therefore a study of their interaction
with nucleons is not possible via direct scattering experiments.
  This interaction is, however, accessible via its influence on the energy
dependence - and on the phase space distributions of the cross sections for
reactions in which these mesons are produced.
  In case of the p p --> p p eta reaction the experimentally determined
distributions of the differential cross sections close to the production
threshold cannot be described by taking into account the S-wave proton-proton
and proton-eta interaction only. Here we show that the angular distributions
determined at the COSY-11 facility reveal some evidence for P-wave admixture in
the proton-proton subsystem already at an excess energy as low as Q = 15.5 MeV.
  We also present that one can estimate the relative strength of the
eta-nucleon and eta-prime-nucleon interactions by comparison of the eta and
eta-prime production yield.
