Form birefringence curves were determined for fixed (and unfixed rat axons before and after lipid extraction. The total detected birefringence was assumed to be due to the macromolecular array of myelin sheath components (phospholipids, cholesterol, and proteins). Unfixed nerves displayed negative form birefringence. Their form birefringence curve exhibited a = refractive index match point positioned at n = 1.46 (intrinsic birefringence). Formalin fixation induced decrease in the optical retardation values but did not affect the profile of the form birefringence curves. After lipid removal, however, the anisotropic patterns of the fixed and unfixed nerves changed. A positive form birefringence was then exhibited, which is attributed to the macromolecular orientation of the protein framework of the myelin sheath. Changes in the shape of the form birefringence curve and in the localization (and number) of the refractive index match point were found. They varied as lipids had been removed from nerves subjected or not to fixation. Therefore, the form birefringence of the myelin sheath proteins plays a part in the total phenomen observed in the whole nerves interfering with that displayed by phospholipids and cholesterol of the mentioned structure.