Acceleration of low-energy protons by quasi-perpendicular shocks is investigated. The pitch angle distribution of ions in the energy range 35 keV to 1 MeV has been determined across the interplaentary shock that passed through the ISEE 3 spacecraft on November 30, 1979. Upstream of the shock a bidirectional angular distribution was observed. It is suggested that multiple crossings of the field line with the surface of the shock, forming a ''magnetic bottle,'' may account for such an unusual angular distribution. The shock event was modeled by integrating particle trajectories numerically. Qualitative agreement between observations and simulations supports the idea of magnetic bottle field line formation. A detailed numerical study of particle acceleration has shown that in the bottle topology the particle flux is enhanced close to the shock front, contrary to the original scatter-free model, i.e., assuming homogeneous magnetic fields on both sides of the shock. It is suggested that multiple crossings of the field line with the shock may explain ''shock spike'' events. ¿American Geophysical Union 1991