A simple, frictional fault model, similar to that introduced by Orowan (1960), is used to show that the apparent stress (product of the seismic efficiency and the mean stress) must be less than one-half the stress drop. A review of the published values of apparent stress and stress drop indicates that within the uncertainties of observation the inequality is generally valid in the magnitude range 1.5 to 8.5 and apparently for all focal depths. The inequality appears to be violated in the case of the Kern County, California, earthquake of 1952 and the aftershocks of the Parkfield, California, earthquake of 1966. The discrepancy for the latter is probably explained by incorrect estimates of the fault parameters. The model also suggests a physical interpretation of the apparent stress; it is simply the stress associated with radiation resistance.