This thesis describes crystallographic investigations into the structures of a number of biological macromolecules. The purpose of these investigations has been to provide an understanding of the structural basis of their biological activities and their interactions with other macromolecules. The first chapter provides a general introduction to this study and, in particular, to the two proteins that were investigated - equine phospholipase A2 and salmon calcitonin. Chapter two provides some general theory into the methods of x-ray crystallography and its application to the solution of the structure of an agonist to 5-HT_1A receptors. Chapter three outlines the bulk purification of another phospholipase A2 from sheep pancreas for crystallographic study. Chapter four outlines the theory and methodology of protein crystallisation as applied to the attempted crystallisation of salmon calcitonin and two derivatives thereof. The crystallisation of a supposed phospholipase A2 inhibitor complex is also reported. Chapter five outlines the process of x-ray structure refinement as applied to equine phospholipase A2 using two refinement packages XPLOR and TNT. The refinement of the proposed enzyme inhibitor complex is also reported. Chapter 6 outlines the three dimensional structure of equine phospholipase A2 and compares and contrasts this structure with a previously determined phospholipase S2 structure. The last chapter, chapter seven, concludes on the results of the various investigations, correlating structure with biological activity.