Monoclonal antibodies were isolated by screening 475 hybridomas obtained from mice immunized with whole leech nerve cords. The majority (about 300) reacted with leech nervous tissue, but only about 40 made antibodies that identified single kinds or small sets of cells. Twenty of the antibodies which react with specific neurones were studied in greater detail and are described here. They include antibodies against identified sensory neurones and motor neurones as well as against numerous unidentified cells.