Antibodies directed against phosphorylated neurofilaments, which are major proteins of the neuronal cytoskeleton, usually do not label neuronal cell bodies except in some neurological diseases. In the present study, we show that in rat cortical cell cultures exposed to kainate there is an inverse relation between neuronal survival and the proportion of neuronal cell bodies stained by a monoclonal antibody (clone SMI31) that recognizes extensively phosphorylated neurofilament proteins (150 kDa and 200 kDa). The immunoblot analysis also revealed an increase in 150-kDa phosphorylated neurofilament expression in kainate-treated cell cultures. Furthermore, the direct quantification of viable neurons SMI31-immunopositive or immunonegative in perikarya showed that the majority of neurons resistant to kainate toxicity expressed phosphorylated neurofilaments in their cell bodies. The percentage of viable neurons displaying SMI31-immunoreactivity in their cell bodies increased from 14.7% in control cultures to 30.0% in cultures treated with 10 microM kainate. These data suggest that phosphorylated neurofilament expression is associated with a reduced cell vulnerability to excitotoxicity induced by kainate.