A monoclonal antibody, GTE52, was isolated from a fusion of myeloma cells with the lymphocytes of a mouse immunised with enzymatically dissociated guinea-pig trigeminal ganglion cells. GTE52 was found to stain the nuclei of satellite cells and Schwann cells, but not neurones, in the peripheral nervous system of guinea-pig and mouse. In the central nervous system GTE52 labelled glia and some neurones. Double-labelling experiments on primary cultures of optic nerve using antibodies to glial fibrillary acidic protein, galactocerebroside and fibronectin showed that GTE52 labelled a sub-population of astrocyte glia, possibly corresponding to the type 2 astrocytes, oligodendrocytes and not fibroblasts. Adult non-neural tissue was not stained by GTE52 with the exception of the smooth muscle of the gut. However, during development of the guinea-pig the antigen recognised by GTE52 was expressed in all cells of 16-day embryos but was lost from the tissues studied, which were not stained in the adult, from about embryonic day 60 onwards.