It has been suggested since the 1970’s that animals like the European robin have a magnetic “compass” sense which is based on magnetic field effects in a suitable chemical reaction. We have proposed experiments that might form the basis of a test for the operation of this “radical pair mechanism” (RPM) compass in a biological system such as the robin. In order to provide a lead for the — often very challenging — experiments on migratory birds performed by our collaborators (R & W Wiltschko and H Mouritsen), I investigated the sources and nature of direction dependence (anisotropy) of the yields of free radical reactions. This work included the development of efficient algorithms to calculate such anisotropies, as well as to quantify their suitability for use in a “compass” receptor. I worked to identify particular effects from Earth-strength static magnetic fields, and how these might be disrupted using low-power RF radiation. We have recently shown the first reproducible magnetic field effects from an Earth-strength field, and the first anisotropic (“compass”) effects at weak magnetic field. This work has been accepted for publication.