A battery of polyclonal and monoclonal antibodies raised against the triplet of identified neurofilament protein subunits was used to investigate neurofilament protein immunoreactivity in neurons of the guinea-pig coeliac ganglion. Using optimal conditions of fixation and tissue processing for each antibody we found that only 20% of the postganglionic sympathetic neurons in the guinea-pig coeliac ganglion contain neurofilament protein-triplet immunoreactivity. Double labelling with neurofilament protein-triplet antibodies raised in different species demonstrated that all of these antibodies labelled the same population of neurons. Double labelling using mouse monoclonal antibodies against neurofilament proteins in combination with rabbit polyclonals to neuronal markers showed that neurofilament protein-triplet immunoreactivity is restricted to specific chemically coded subpopulations of noradrenergic neurons. Approximately 52% of neurons in the ganglion contain neuropeptide Y and are presumed vasomotor neurons projecting to blood vessels in the submucosa of the small intestine. Virtually none of the neuropeptide Y-containing neurons were labelled with neurofilament protein-triplet antibodies. Neurons that contain somatostatin (21%) project to the submucous ganglia of the small intestine. Approximately two-thirds of neurons containing somatostatin are immunoreactive for the neurofilament protein-triplet. The other postganglionic neurons in the ganglion (27%) project to the myenteric plexus of the small intestine and do not contain either neuropeptide Y or somatostatin. Approximately a quarter of these neurons were labelled with neurofilament protein-triplet antibodies. These results suggest that the neurofilament protein-triplet may not be an instrinsic component of the cytoskeleton of all neurons. Furthermore the idea of a chemical coding of neurons should be extended to cytoskeletal proteins. The finding that these neurofilament proteins are confined to specific neuronal subpopulations has important implications for the search for a role of the neurofilament protein-triplet in neurons, for the interpretation of classical neurohistological silver impregnation techniques which appear to stain only neurofilament protein-triplet-containing neurons, as well as for neuropathological conditions that may involve these proteins in disease processes.