Strange particle production in proton-neutron collisions is calculated by means of the one-pion exchange (O.P.E.) model for an incoming neutron with a kinetic energy of 3 GeV in the laboratory frame. The calculation takes into account two Feynman diagrams and includes the interference term. Use is made of the Ferrari-Selleri cut-off functionC(α,q2) and these results are compared with those that do not take this into account. The lab. energy spectra of the outgoing nucleons are drawn for the six p n processes, and their total cross-sections evaluated. From these results the total cross-section values for the four corresponding pp reactions are deduced and compared with their known experimental values at 2.85 GeV. Good agreement is shown in three of these cases, but the theoretical result for the fourth one is somewhat small.