We characterized an antibody raised against a peptide derived from the ALS2 protein, the product of a gene linked to recessive juvenile forms of amyotrophic lateral sclerosis, defining the ALS2 RLD-derived antigen (ARDA). In cultured rat cortical neurons, ARDA is localized in perikarya and neurites, but absent from nuclei. Double immunostaining of rodent cultured neurons and spinal cord sections for ARDA and MAP2, a marker of dendrites, or tau and phosphorylated neurofilaments, markers of axons, demonstrated that the ARDA antigen was localized in the somatodendritic compartment and excluded from axons. ARDA was also selectively localized to the somatodendritic compartment of motor neurons in human spinal cord. The ARDA antigen therefore represents a novel somatodendritic marker for embryonic and adult neurons.