A series of monoclonal antibodies that distinguish phosphorylated and nonphosphorylated neurofilament (NF) epitopes was used to immunostain brain stem neurons from control rabbits and from rabbits chronically intoxicated with Aluminium (Al). In controls, none of the monoclonal antibodies to phosphorylated NF stained the perikaryon of neurons. In contrast, in animals treated with Al, all neuronal perikarya containing Al-induced neurofilament bundles (NB) and some lacking well-formed NB immunoreacted with two of the five antibodies to phosphorylated NF. Axons were stained by all five antibodies to phosphorylated NF in both control and Al-treated animals. A broadly reacting monoclonal antibody to a nonphosphorylated NF epitopes immunoreacted with neuronal cell bodies, dendrites and axons in control and Al-intoxicated animals regardless of the presence of Al-induced NB. Staining of Al-induced NB with one of the antibodies to phosphorylated NF was greatly diminished after treatment of sections with trypsin and phosphatase. It is concluded that NF which compose the Al-induced NB have different immunocytochemical characteristics from those of the NF present in the perikaryon of normal neurons. It is likely that, contrary to normal perikaryal NF, NF of Al-induced NB are phosphorylated. Moreover, phosphorylation of NF of Al-induced NB is probably abnormal, since NF of Al-induced NB have immunostaining characteristics different from NF of normal axons. Al-induced NB may result from abnormal phosphorylation of NF in the perikaryon, preventing normal axonal transport of these structures.