We report the first observation of antiferromagnetic resonance in La2-xCuO4-y. Transmitting far-infrared radiation through our polycrystalline samples we observe a resonance at 9 cm-1 from which we extract an anisotropy energy of 0.02 cm-1. This is more than an order of magnitude smaller than in the quadratic-layer spin-1 antiferromagnet K2NiF4, in which anisotropy has been shown to be responsible for the observed Néel transition. Nevertheless, using the same analysis we find that, because of the very large intraplanar exchange coupling, the small anisotropy that we measure is sufficient to account for the observed Néel temperatures of up to 300 K in La2-xCuO4-y. © 1988 The American Physical Society.