Accurate birefringence measurements show that fibrinogen orients to a small degree in high magnetic fields. This effect can be explained as due to the molecule having about 30% (by weight) alpha-helix oriented relatively parallel to the long axis. Birefringence measurements on fully oriented fibrin suggest that aligned alpha-helical content is less than that estimated for fibrinogen. But because of limitations in the analysis this difference must be viewed with caution. Highly oriented fibrin results when polymerization takes place slowly in a strong magnetic field. Low-angle neutron diffraction patterns from oriented fibrin made in the presence of EDTA, made in the presence of calcium, or stabilized with factor XIIIa are very similar, showing that the packing of the molecules within the fibers is the same or very similar in these different preparations. The induced magnetic birefringence was used to follow fibrin formation under conditions in which thrombin was rate limiting. The fiber network formed by approximately the gelation point constitutes a kind of matrix or frame that is largely built upon during the remaining approximately 85% of the reaction. After gelation the reaction is pseudo-first order.