Reduction of structure to parabolic subgroups

. Let G be an affine group over a field of characteristic not two. A G -torsor is called isotropic if it admits reduction of structure to a proper parabolic subgroup of G . This definition generalizes isotropy of affine groups and involutions of central simple algebras. When does G admit anisotropic torsors? Building on work of J. Tits, we answer this question for simple groups. We also give an answer for connected and semisimple G under certain restrictions on its root system.