Short‐duration increases in particle flux at 1‐Mev energy, detected by Explorer 34 just before the time of passage of interplanetary shocks are discussed. The effect is interpreted as particle acceleration associated with the shocks; most of the increase is found to be consistent in many ways with the idea of energy gain by successive reflection between the earth's bow shock and the interplanetary shock. Other mechanisms are not excluded, however. From the small sample observed, it appears that shocks occurring when the interplanetary magnetic field at the observer's position does not intersect the earth's bow shock did not show particle flux increases, and that the particles accelerated to 1 Mev by the observed shocks did not come from solar wind energies, but rather from solar particles in the hundred‐kev energy region which were available in the m