In the coming year we plan to pursue 4 lines of research: a) Electron microscopy of thin sections of fibrin. We will use tannic acid fixation in an attempt to visualize the packing of protofibrils in cross-sections of fibrin fibers. b) Electron microscopy of Factor VIII. These studies will be pursued in collaboration with Dr. P.A. McKee's lab for purification and characterization of Factor VIII. c) Electron microscopy of fibronectin. This is a serum protein closely related to the cell surface fibronectin. We have already observed the shape of the molecule (a long flexible chain) and its attachment to fibrinogen molecules. d) Nucleation and assembly of fibrinogen and fibronectin. We will use a combination light scattering and electron microscopy to characterize small polymers of these two proteins, and especially to investigate the question of nucleation of polymerization.