The floor plate is a cell group implicated in the control of neural cell pattern and axonal growth in the developing vertebrate nervous system. To identify molecules that might mediate the functions of the floor plate, we have used subtractive hybridization techniques to isolate floor plate-enriched cDNA clones. One such clone encodes a novel secreted protein, F-spondin, which is expressed at high levels in the floor plate. The C-terminal half of the protein contains six repeats identified previously in thrombospondin and other proteins implicated in cell adhesion. F-spondin is expressed in the floor plate at the time that axons first extend and at lower levels in the peripheral nerve. Recombinant F-spondin promotes the attachment of spinal cord and sensory neuron cells and the outgrowth of neurites in vitro. F-spondin may contribute to the growth and guidance of axons in both the spinal cord and the PNS.