An object is considered chiral if its mirror image cannot be brought to coincide with itself by any sequence of simple rotations and translations 1 . Chirality on a microscopic scale - in molecules 2,3 , clusters 4 , crystals 5 and metamaterials 6,7 - can be detected by differences in the optical response of a substance to right- and left-handed circularly polarized light 2,3 . Such 'optical activity' is generally considered to be a consequence of the specific distribution of electronic charge within chiral materials. Here, we demonstrate that a similar response can also arise as a result of spin excitations in a magnetic material. Besides this spin-mediated optical activity (SOA), we observe notable differences in the response of Ba 2 CoGe 2 O 7 - a square-lattice antiferromagnet that undergoes a magnetic-field driven transition to a chiral form - to terahertz radiation travelling parallel or antiparallel to an applied magnetic field. At certain frequencies the strength of this magneto-chiral effect is almost complete, with the difference between parallel and antiparallel absorption of the material approaching 100%. We attribute these phenomena to the magnetoelectric nature of spin excitations as they interact with the electric and magnetic components of light.