  We report on measurements of the magnetic response of the anisotropic CuO_2
spin chains in lightly hole-doped La_x (Ca,Sr)_14-x Cu_24 O_41, x>=5. The
experimental data suggest that in magnetic fields B >~ 4T (applied along the
easy axis) the system is characterized by short-range spin order and
quasi-static (quenched) charge disorder. The magnetic susceptibility chi(B)
shows a broad anomaly, which we interpret as the remnant of a spin-flop
transition. To corroborate this idea, we present Monte Carlo simulations of a
classical, anisotropic Heisenberg model with randomly distributed, static
holes. Our numerical results clearly show that the spin-flop transition of the
pure model (without holes) is destroyed and smeared out due to the disorder
introduced by the quasi-static holes. Both the numerically calculated
susceptibility curves chi(B) and the temperature dependence of the position of
the anomaly are in qualitative agreement with the experimental data.
