Lewis rats were immunized with P2 protein and peptides CN1, CN2 and CN3. In P2 immunized rats, antibody and cellular sensitivity to the P2 protein were seen prior to the appearance of clinical signs of EAN, during disease and during recovery. CN1 immunized rats had equal or more severe clinical and histological signs of EAN but lower antibody titers than P2 immunized animals. CN-1 elicited a greater blastogenic response in both P2 and CN-1 immunized animals. CN-2 and CN-3 immunized rats showed little clinical and histological evidence of EAN and no antibody. However, lymphocytes sensitive to the immunizing antigen and the P2 protein were present.