It has been suggested that chemical reactions proceeding through radical pair intermediates could form the basis of bird's ability to sense the geomagnetic field as a source of compass information [Biophys. J. 78 (2000) 707]. We present calculations of anisotropic reaction product yields for a flavin-tryptophan radical pair subject to a magnetic field of ∼50 μT. The anisotropic response of the reaction is found to be dominated by two nitrogen nuclei in the flavin radical which have near-axial hyperfine interactions with almost collinear principal axes. It is shown that the anisotropy of the product yields is not strongly dependent on the lifetime of the radical pair in the range 1-5 μs, and that it can be tuned by small variations in the hyperfine tensors of the nuclear spins in the two radicals. © 2003 Elsevier B.V. All rights reserved.