We present one example of correlated particle and magnetic field observations of a large-scale magnetic loop structure taken from a survey of observations on ISEE-3. Approximately 12 hours after the passage of the interplanetary shock of 11 December 1980 and directly after the passage of a discontinuity, bedirectional proton fluxes were observed for a period of 40 hours in the energy range 35--1600 keV. In each of the eight logarithmically spaced energy channels, a three-dimensional anisotropy analysis showed streaming along both directions of the magnetic field. During the same 40-hour period, the magnetic field rotated slowly but steadily through approximately 180¿, consistent with the passage of a loop structure. This event is unique, in that both the particle and field measurements are clearly consistent with the existence of a large-scale loop, with extent greater than 0.5 AU. Our observations indicate that the particles are being injected into the loop sunward of the spacecraft, and appear as bidirectional fluxes in the outermost regions of the loop due to a combination of focussing and near scatter-free transport.