Abstract β,β′-Iminodipropionirtrile (IDPN) is known to produce a massive accumulation of neurofilaments in the proximal portion of axons of spinal anterior horn cells. The spinal cords of 20 Wistar rats treated with IDPN were examined immunohistochemically with a monoclonal antibody against heparan sulfate (HepSS-1). Virtually all axonal swellings were intensely labeled with HepSS-1. This immunoreactivity was almost completely absorbed by the presence of CDSNS-HS (completely desulfated, N-sulfated heparan sulfate). Western blot study revealed that HepSS-1 recognized four distinct bands at the positions of ∼17 kDa, ∼20 kDa, ∼21 kDa, and ∼25 kDa. The present study suggests that the deposit of heparan sulfate in spheroids is related to the pathomechanism for the formation of axonal swelling.