Conventional antibodies have long been used in an attempt to produce specific neural markers. Such markers would be invaluable for studying the structural organization and development of the nervous system. Unfortunately, they have not been found to discriminate satisfactorily between neuronal subpopulations. Recent developments of the hybridoma technique, however, promise to provide monoclonal antibodies of adequate specificity. Such antibodies have already generated and shown to be capable of distinguishing between individual neurones of the leech nervous system. We report here two monoclonal antibodies which, although generated against human T cells, react exclusively with Purkinje cells in the vertebrate central nervous system (CNS). This new specificity arose out of a fortuitous observation made during examination of the lymphocyte infiltration of human cerebellar tumours with the monoclonal antibody, UCHT1. Although widely used as a T-cell marker, its reaction with neural tissue has not hitherto been described. To our knowledge, this is the first description of a monoclonal antibody which recognises discrete neuronal population in the vertebrate brain.