In mice that express lacZ under the control of a human dopamine beta-hydroxylase gene promoter (DbetaH-nlacZ mice), the nuclei of enteric neurons express the transgene, as shown by the presence of beta-galactosidase (beta-gal) staining (Mercer et al.  Neuron 7:703-716). The transgene is also expressed by neural crest-derived cells in the developing gut before their differentiation into neurons or glial cells (Kapur et al.  Development 116:167-175). However, the cell types expressing the DbetaH-nlacZ transgene within the developing and adult gut have not been fully characterized. Whole-mount preparations of embryonic and adult gut were processed for histochemistry or immunohistochemistry to reveal beta-gal plus markers of undifferentiated neural crest cells (in embryos) or enteric neurons (in adults). In embryonic mice, over 90% of undifferentiated neural crest-derived cells (identified using antibodies to p75) were beta-gal(+). Importantly, crest-derived cells at the migratory wavefront were all beta-gal(+). In adult mice, only a subpopulation of enteric neurons was beta-gal(+), while glial cells showed no beta-gal staining. Considerable variation was observed between the small intestine and colon in the proportion of myenteric neurons that showed beta-gal staining. We examined whether known classes of enteric neurons varied in their expression of DbetaH-nlacZ. In the myenteric plexus of the jejunum and colon, large calretinin(+) neurons did not express lacZ, suggesting that the incomplete penetrance of the DbetaH-nlacZ transgene observed in adult mice is not random. We conclude that the DbetaH-nlacZ transgene provides a reliable marker for examining the colonization of the developing gut by neural crest cells. However, in adult mice, there is variation between mice, between gut regions, and between different classes of enteric neurons in the expression of the transgene.