The P2 protein of bovine nerve root myelin was radiolabeled with 125I in homogeneous solution by the chloramine-T method and purified by gel filtration on Sephadex G-75 to obtain monomeric 125I-P2. Antigen-antibody complexes were isolated by the silica gel, double antibody, or polyethylene glycol techniques by using rabbit antibody to P2. As little as 1 ng/ml P2 could be detected. The RIA was used to measure the P2 content in nerve tissue and isolated myelin. The presence of P2 in spinal cord as well as in the peripheral nervous system was confirmed. Peptides isolated by CNBr digestion of the P2 protein were tested in the RIA. CN-1, comprising 80 to 90 residues from the interior of the molecule displayed complete immunologic cross-reactivity with intact P2. Neither CN-2, representing 18 amino acids from the COOH terminal, nor CN-3, representing 20 amino acids from the NH2 terminal, showed cross-reactivity. Since the major determinant for experimental allergic neuritis in the rabbit is located in peptide CN-2, our present data suggest that the major neuritogen and the major determinant(s) for humoral antibody response in this species may be at different locations within the P2 molecule.