  We investigate the influence of biquadratic exchange interactions on the
low-lying excitations of a S=1/2-ladder using perturbation theory, numerical
diagonalization of finite systems and exact results for ladders with matrix
product ground states. We consider in particular the combination of biquadratic
exchange interactions corresponding to ring exchange on the basic ladder
plaquette. We find that a moderate amount of ring exchange reduces the spin gap
substantially and makes equal bilinear exchange on legs and rungs consistent
with experimentally observed spectra.
