  We study the properties of one-dimensional gapped Heisenberg antiferromagnets
in the presence of an arbitrary strong staggered magnetic field. For these
systems we predict a universal form for the staggered magnetization curve. This
function, as well as the effect the staggered field has on the energy gaps in
longitudinal and transversal excitation spectra, are determined from the
universal form of the effective potential in O(3)-symmetric 1+1--dimensional
field theory. Our theoretical findings are in excellent agreement with recent
neutron scattering data on R_2 Ba Ni O_5 (R = magnetic rare earth) linear-chain
mixed spin antiferromagnets.
