A monoclonal antibody (RT97) against neurofilament protein specifically and exclusively labelled a subpopulation of rat dorsal root ganglion (DRG) neurones. For seven ganglia (L4 and T13) studied quantitatively the frequency distribution histograms of the size of labelled cells could be fitted by a single normal distribution whose parameters were extremely close to those of the normally distributed large light cell population in that ganglion. On this basis and on the basis of a statistical analysis of the results it was suggested that this antibody can be used as a much needed specific label for the large light population of neurones in rat DRGs. The small dark neurone population was not labelled by this antibody. In one ganglion the subjective analysis of whether each neurone was labelled or not was directly compared with microdensitometric measurements of reaction product intensity. This analysis supported the above conclusion, and furthermore no subdivisions of the labelled population were apparent on the basis of neuronal size plotted against intensity of the reaction product. Other neuronal cell bodies strongly labelled by this antibody were found in association with small unlabelled neurones not only in DRGs, but also in the trigeminal ganglion, the vagal ganglia, and the mesencephalic V nucleus, all of which are made up of primary afferent neurones and all of which are completely or partially derived from the neural crest. Sympathetic and central nervous system neuronal cell bodies were unlabelled or occasionally very lightly labelled although immunoreactive fibres abound in the central nervous system.