We present orbit-averaged geomagnetic transmission measurements during the large solar energetic particle events of October 1989 using proton data from the NOAA-10 and GOES-7 satellites. We compare the measurements to geomagnetic transmission calculations determined by tracing particle trajectories through the combination of the International Geomagnetic Reference Field (IGRF) model and the 1989 Tsyganenko magnetospheric magnetic field model. We modify the effective 'ring current' parameter in the 1989 Tsyganenko model based on the Dst data. We compare our results to calculations employing only the IGRF and to a parameterization of geomagnetically quiet-time cutoff rigidities derived from Cosmos/Intercosmos observations. Our 3-hour orbit-averaged results have ~15% accuracy during the October 1989 events. ¿American Geophysical Union 1995