In order to assess the potential of protein gene product (PGP) 9.5 as a marker of the nervous and neuroendocrine systems, we examined its immunolocation in human, rat and guinea-pig tissues, using a rabbit polyclonal antiserum and two new mouse monoclonal antisera, I3C4 and 3IA3. Our results demonstrate immunoreactive PGP 9.5 in neurons and nerve fibres at all levels of the central and peripheral nervous system, in many neuroendocrine cells, in part of the renal tubule, in spermatogonia and Leydig cells of the testis, and in ova and in some cells of the pregnant and non-pregnant corpus luteum. In routinely processed tissues, standard immunohistochemical techniques using the polyclonal antibody demonstrated peripheral nerve fibres of all sizes with striking clarity.