Abstract We have previously shown that neurofilaments in enteric neurons are immunologically distinct from those found in the central nervous system. In particular, one monoclonal antibody to the medium molecular weight neurofilament subunit, called NN18, stained the perikarya of enteric neurons very weakly, if at all, although other medium molecular weight neurofilament subunit antibodies clearly showed the presence of significant amounts of medium molecular weight neurofilament subunit. We have since located the epitope for NN18 in a peptide sequence at the car☐y terminal tail of medium molecular weight neurofilament subunit and have now looked for further neurofilament antibodies that stain the same region. We found two monoclonal antibodies, RMO1 and RMO59, that recognize the same peptide. These antibodies also show much reduced staining in myenteric neurons compared to those in the central nervous system, suggesting that this region of the medium molecular weight neurofilament subunit is specifically modified in certain cells. In the developing enteric nervous system, we found that all the antibodies show strong staining of enteric neurons in the neonatal rat, but that with NN18, RMO1, and RMO59, the staining intensity decreases during further development, and by postnatal day 21 all three demonstrate decreased or absent staining identical to the adult. These results indicate that myenteric neurons, in contrast to the brain, have a developmentally regulated modification in a specific region of the medium molecular weight neurofilament subunit, which may reflect adaptation to structural stress by myenteric neurons.