We are studying a mannose-specific recognition mediating the projection of axons in the synaptic neuropil of the embryonic leech CNS. A functional class of neurons, the sensory afferents, can be distinguished by a mannose-containing epitope that is asparagine-linked to a 130 kDa surface protein and is reactive with the monoclonal antibody Lan3-2. Sensory afferents project as a tightly fasciculated bundle through peripheral nerves but, upon arriving in the CNS, defasciculate into the synaptic neuropil. This defasciculation allows the previously bundled sensory afferents to form an arborization in the synaptic neuropil. Three lines of experimental evidence indicate that the defasciculation is mediated by the sensory afferent's mannose- containing Lan3-2 epitope. The defasciculation is inhibited (1) by blocking the Lan3-2 epitope with Lan3-2 Fab fragments, (2) by cleaving the asparagine-linked carbohydrate moieties from surface proteins with the glycosidase N-glycanase, and (3) by competing for a putative mannose-binding protein with the neoglycoprotein mannose-BSA [albumin, p-aminophenyl alpha-D-mannopyranoside (26 mol monosaccharide/mol albumin)]. In addition to inhibiting the defasciculation, the three perturbation reagents also elicited the refasciculation of axons that had defasciculated prior to their application. These three different experimental approaches provide strong evidence that carbohydrate recognition regulates the projections of sensory afferents in the leech synaptic neuropil. Carbohydrate interactions therefore can play a major role in regulating the neuronal architecture in the CNS.