Experiments and theory are combined in an attempt to determine how well a demagnetizing factor, N, characterizes the internal magnetic field of a multidomain (MD) grain. lt is shown that all previous experimental estimates of N are suspect and that only under unusual circumstances can one have much confidence in determining a value for N that characterizes the internal magnetic field. One of those circumstances is when the sample is nearly stoichiometric, as is the case of a large synthetic magnetite crystal found here to have an MD value of N essentially identical to its single-domain value. However, calculations show that the value for N in small MD grains depends on the domain and magnetization state of the sample. It is shown that the demagnetizing factor cannot be uniquely determined for small MD grains, but it can often be well approximated by the single-domain demagnetizing factor for large MD grains. A value of 10 μm is shown to be roughly the linear dimension that separates ''large'' from ''small'' grains in the case of equidimensional magnetite. ¿American Geophysical Union 1987