The bovine P2 protein is known to produce experimental allergic neuritis (EAN) in Lewis rats. The longest region of amphipathic alpha-helix within the bovine P2 protein was identified as residues 61-72. A synthetic peptide representing this sequence was synthesized and shown to have the ability to (1) produce EAN in Lewis rats by direct sensitization, (2) produce a lymphoproliferative response in a P2-specific T cell line, and (3) stimulate a P2 protein-specific T cell line sufficiently to transfer EAN to naive recipient Lewis rats. Residues 61-72 is the shortest sequence thus far identified that will produce EAN in the Lewis rat and supports the contention that T cell determinants tend to be amphipathic alpha-helices.