Primary sensory olfactory neurons reside in a neuroepithelium lining the nasal cavity and project topographically onto the surface of the olfactory bulb, a rostral extension of the telencephalon. Galectin-1, a bivalent galactose-binding vertebrate lectin, is expressed in the developing rodent olfactory system. In the present study, the mouse olfactory neuron cell line 4.4.2 was used to examine the role of galectin-l in neurite outgrowth in vitro. Recombinant galectin-l has neurite outgrowth-promoting activity when used as a substrate for 4.4.2 cells. When either galectin-1 or lactose was added to the culture media, the neurite outgrowth-promoting activity was abolished. These results demonstrate that galectin-1 can modulate neurite growth in vitro. The in vivo role of galectin-1 was investigated by examining the topographical organization of the olfactory pathway in mice carrying a null mutation for galectin-1. Using Dolichos biflorus agglutinin as a convenient histochemical marker of a subpopulation of primary sensory olfactory neurons which project topographically to the dorsomedial olfactory bulb, we show an aberrant topography of olfactory axons in the null mutants. A subset of primary sensory olfactory axons failed to project to their correct target sites in the caudal olfactory bulb. These data indicate that galectin-1 is involved in the growth and/or guidance of primary sensory olfactory axons between the nasal cavity and the olfactory bulb. This is the first demonstration that a lectin has neurite outgrowth-promoting activity and plays a role in neuronal pathfinding in the mammalian nervous system.