We have used a crossreactive monoclonal antibody against a mouse neural cell adhesion molecule (N-CAM) to characterize and purify an antigen from embryonic and neonatal human brain. This antigen was identified as human N-CAM according to several criteria, including molecular weight, sialic acid content, amino acid composition, analysis of peptide fragments, reactivity with a variety of anti-CAM antibodies, and activity of the antigen in assays of cell and membrane vesicle aggregation. The findings make feasible an exploration of the possible role of N-CAM in various neurological disorders.