The distribution of binding of the plant lectin soybean agglutinin to primary sensory neurons has been investigated in the rat and cat. Soybean agglutinin was found to bind to approximately 30% of neurons in the fourth, lumbar dorsal root ganglion of the rat and approximately 50% of neurons in the first, sacral dorsal root ganglion in the cat. Morphometric analysis of these dorsal root ganglia revealed that in both species those neurons which bind soybean agglutinin appear to be a subpopulation of the small-diameter cells. Electron microscopic analysis of the lumbar dorsal roots revealed that soybean agglutinin binds to the plasma membrane of a subpopulation of unmyelinated axons in both species. Myelinated axons did not bind the lectin. In the dorsal horn of the spinal cord, soybean agglutinin bound to components of Lissauer's tract and the superficial laminae (laminae I and II) of the dorsal horn in both species. In the dorsal horn lectin binding was limited to lamina I and the outer portion of lamina II (sublamina IIo) in the cat, while in the rat lamina I and the entire dorsoventral extent of lamina II (sublaminae IIo and IIi) were labelled. Thirty days following dorsal rhizotomy, soybean agglutinin binding in the superficial dorsal horn, ipsilateral to the rhizotomy, was abolished in both species. These results show the plant lectin soybean agglutinin to be a histochemical marker for the soma and central terminals of a subpopulation of small-diameter sensory neurons in both the rat and cat. It is suggested that soybean agglutinin binding may be used in conjunction with immunohistochemistry to allow identification of putative neurotransmitters within, or in synaptic profiles associated with, the central terminals of small-diameter primary sensory neurons.