Antiferromagnetic resonance has been observed in powdered MnO at 364 &mgr; at 1.5°K. The resonancefrequency as a function of temperature falls roughly as the square root of the Brillouin function of spin52.Fields of the order of104oe have no observable effect on the line at 1.5°K. All of these results give quantitativeconfirmation of a model in which the sublattice magnetizations are constrained to a (111) plane bymagnetic dipolar interactions and are held to a preferred direction within the plane by a much weakeranisotropy.