The mouse monoclonal antibody RT97, which recognises the 200-kDa neurofilament subunit in its phosphorylated form, selectively labels the somata of sensory A-fibres (large light cells) in the dorsal root ganglion of the rat. We have tested the hypothesis that this antibody also visualises large diameter sensory fibres and their end structures in peripheral tissue, in particular in the skin. RT97 immunoreactivity is found in endings that are known to be served by myelinated afferent fibres, including Meissner-like endings, Merkel discs, hair follicle receptors, Pacinian corpuscles and free nerve endings. RT97 immunoreactivity has not, however, been observed in endings of presumably unmyelinated sensory fibres (intraepidermal fibres immunoreactive for substance P and calcitonin gene-related peptide) or in sympathetic fibres innervating sweat glands and blood vessels. In addition, neither systemic (100-150 mg/kg as adults) nor perineural capsaicin pre-treatment affects RT97 immunoreactivity in the skin. The data indicate that RT97 is a useful marker in the study of the capsaicin-insensitive sensory innervation of the skin and possibly other peripheral organs.