1. Axoplasm of squid giant nerve fibers is examined with the ultracentrifuge and electrophoresis apparatus and several distinct components demonstrated. 2. One of these components, a protein called axon filaments, is isolated by fractional extraction followed by differential ultracentrifugation and redissolving in glycine solution. Axon filaments are monodisperse by ultracentrifugation. Their physical chemical properties have been studied. 3. The existence of a reversible transformation of axon filaments into a particle of lower molecular weight and lower asymmetry has been demonstrated.