The largest and best characterised family of neurotrophic growth factors is that of nerve growth factor (NGF) and its relatives. In order to understand the relation of structure and function, we have undertaken X-ray analyses of murine NGF. The active component beta-NGF crystallises as hexagonal bipyramids that give good X-ray diffraction data using a synchrotron to 2.3A resolution. We have prepared several heavy atom derivatives that are being used in the method of multiple isomorphous replacement to solve the phase problem and determine the three-dimensional structure. We have also prepared crystals of the precursor, 7S NGF, which is a complex of three different subunits of composition alpha 2 beta 2 gamma 2. We have collected X-ray data to 3A resolution on two crystal forms with related cell dimensions and orthorhombic spacegroups. Detailed analyses of the structures of NGF in these crystal forms, taken together with data on sequence and biological activity, should give clues concerning the role of the precursor complex in storage and assist the identification of the surface region involved in receptor binding.