The guidance of axons to their targets in the developing nervous system is believed to involve dlffuslble chemotroplc factors secreted by target cells. Floor plate cells at the ventral midline of the spinal cord secrete a diffusible factor or factors that promotes the outgrowth of spinal commlssuml axons and attracts these axons In vitro. Two membrane-associated protelns isolated from brain, netrln-1 and netrln-2, possess commlssuml axon outgrowth-promoting activity. We show here that nefrln-7 RNA is expressed by floor plate cells, whereas netrln-2 RNA Is detected at lower levels in the ventral two-thirds of the spinal cord, but not the ftoor plate. Heterologous cells expressing recombinant netrln-1 or netrln-2 secrete diffusible forms of the proteins and can attract commissuml axons at a distance. These results show that netrln-1 is a chemotropic factor expressed by floor plate cells and suggest that the two netrln proteins gulde commissum1 axons in the developing spinal cord.