NC-1 and HNK-1, two mouse monoclonal antibodies raised against quail ciliary ganglion and a human leukemic cell-line, respectively, were found to display the same pattern of reactivity. Species investigated included human, rodents, birds and amphibians. In 1- to 3-day-old avian embryos, migrating crest cells are stained, whereas in older animals neuroepithelial cells and neurons are labelled. Staining with fluorescein isothiocyanate-conjugated NC-1 is blocked by preincubation with HNK-1. In immunoblot analyses both antibodies recognize the same pattern of bands which are different in central and peripheral nervous systems and vary during development. Thus, HNK-1/NC-1 provides a useful tool for investigating the ontogeny of neural and lymphocytic cells carrying this determinant, which, in view of its high degree of conservation during vertebrate evolution, may play an important part within the haematopoietic and nervous systems.