The plant lectin Bandeiraea simplicifolia I-B4 binds to the soma and central terminals of a subpopulation of unmyelinated primary sensory neurones in the adult rat. The binding site of this lectin is thought to be the terminal alpha-D-galactose residue of a membrane associated glycoconjugate which may be involved in the development of specific connections between small diameter primary sensory neurones and second order neurones in the superficial dorsal horn of the spinal cord. To begin to investigate this possibility we have examined the development of lectin binding in the dorsal horn of pre- and postnatal rats. Lectin binding first appeared on axon profiles in the superficial dorsal horn of the spinal cord at embryonic days 18/19. Previous studies in the rat have revealed that the central processes of small diameter primary sensory neurones enter the dorsal horn at embryonic days 18/19. Our findings suggest that the glycoconjugate to which this lectin binds, is expressed by the central processes of small diameter primary sensory neurones as they grow into the spinal cord. It is therefore possible that this glycoconjugate is involved in the development of topographically ordered neural connections within the dorsal horn of the spinal cord.