O(4) and Forward Production Processes at High Energies

A method is sketched for writing down invariant amplitudes which give the leading contributions of families of Regge trajectories of definite O(4) class (at zero momentum transfer). The method is illustrated by the case $\pi +N\to \rho +N$, where it leads to definite predictions (in spite of unequal masses) for the leading term in each helicity amplitude.