The HNK-1 glycoepitope, carried by many cell recognition molecules, is present in the developing posterior lateral line nerve and on other primary axons of zebrafish. To elucidate the function of HNK-1 in vivo, the antibody 412 to HNK-1 was injected into zebrafish embryos at 16 h post fertilization (hpf). The injected antibody bound specifically to axons carrying HNK-1. This treatment selectively affected the growth of either one or both posterior lateral line nerves in 39% of the experimental cases (13 of 33 animals), which was significantly more (P < 0.0002) than in uninjected, vehicle injected, and non-immune IgG injected controls (1.2% of the animals; one of 85 animals), as assessed at 27 or 33 hpf. Other HNK-1 immunoreactive nerves, such as the ventral motor nerves were unaffected, indicating that antibody binding per se did not interfere with axon growth. The primordium of the posterior lateral line was not affected in its caudal migration and in depositing differentiating neuromasts along the trunk, showing that injections did not retard development and that initial formation of lateral line organs is probably independent of contact with nerve fibers. We suggest that the HNK-1 glycoepitope is an important modulator of embryonic nerve growth.