Monoclonal antibodies were used to map the distribution of neurons in the leech which contain a particular antigen. This technique reveals the genetically determined variation in cell body distribution along the nerve cord. In addition, antibodies also reveal developmental deviations, such as the occurrence of supernumerary cell bodies. Three antibodies that bind either to single types or small sets of different neurons are used to construct complete distribution patterns of antigenically related cells. Three other antibodies are used to create cell body distribution maps of antigenically homologous primary mechanosensory cells responding to noxious or pressure stimulation which form a subset of the cells stained by the antibody. Furthermore, antibodies against the pressure cells helped in the location of two different specific antigens for the same identified nerve cell.