The stellate ganglion, which gives rise to the giant axons of the squid, was dissected into two parts, one containing primarily cell bodies and the other axon initial segments. A neurofilament protein-enriched extract of each was prepared and compared biochemically and immunochemically with an axoplasmic neurofilament preparation and with the glial sheath that surrounds the axons. Both parts of the ganglion lacked the 220 kDa subunit of axoplasmic neurofilaments (NFs). However, they did contain a protein of about 190 kDa that reacted with the Pruss anti-intermediate filament antibody (aIFA; Pruss et al.: Cell 27:419-428, 1981), but not with a phosphorylation-dependent NF antibody (Cohen et al.: J Neurosci 7: 2056-2074, 1987). Dephosphorylation of the axoplasmic NF220 yielded a product that comigrated on two-dimensional (2D) gel electrophoresis with the 190 kDa ganglion protein, suggesting that the latter represented the incompletely phosphorylated precursor of NF220. The major low molecular weight aIFA-reactive species in the ganglion preparations was a polypeptide of about 65 kDa. A relatively small quantity of that polypeptide was also found in axoplasm and it comigrated in 2D gels with an aIFA-reactive polypeptide from the glial sheath. These results indicate that the site of modification of the 190 kDa NF precursor to the 220 kDa axonal form is probably at the point where the axon initial segments leave the ganglion, which is several mm distal to its site of synthesis in the cell body. Furthermore, the filament network of the axoplasm and possibly the cell bodies includes a glial-like intermediate filament protein in addition to the NF protein subunits.