The HNK-1 antigen, a carbohydrate moiety bound to many cell adhesion and recognition molecules, is implicated in cell-cell and cell-substrate interactions during neural development. HNK-1 immunoreactivity (HNK1-IR) appears on neurons of the Xenopus neural tube very early in their development (Nordlander, Devel. Brain Res., 50:147-153, 1989). The distribution and onset of expression of the HNK-1 epitope on and within individual neurons is examined in this study. HNK-1 labels developing neurons and their processes, and focal areas of other structures which are directly contacted by neurons, such as neuroepithelial cell surfaces, basal lamina, and culture surfaces. HNK1-IR first appears in the Golgi apparatus and subsequently on the cell surface and in streams of punctate material directed toward the site of axon initiation and into the developing axon and its growth cone. The entire neuron is coated with a thin (20-30 nm) surface layer of HNK1-IR. In addition, the surface is dotted with small (100-250 nm) boluses of HNK1-IR material. Such boluses also occur within cytoplasmic vesicles, and extracellularly on basal lamina and culture substrata in proximity to neurons or their processes. The subcellar distribution of HNK1-IR in this tissue is compatible with a role for the HNK-1 epitope in axonal outgrowth and guidance.