It has recently been demonstrated that cellular responses in cattle chronically infected with M. paratuberculosis are regulated by nonT/nonB (N) lymphocytes of the N2+ subpopulation which suppress antigen specific T helper (BoCD4+) lymphocytes. The inactivation of the helper cell population required for macrophage activation may be the underlying mechanism by which M. paratuberculosis is able to replicate. It was also shown that immunity could be expressed by the emergence of a new BoCD4+ population in conjunction with BoCD8+ contrasuppressor cells. Examination of BoCD4+ PBMC and BoCD4+ cell lines illustrated that cellular reactivity was directed, almost exclusively, to a cell wall associated antigen fragment of M. paratuberculosis. Helper reactivity to this antigen is MHC-restricted, requiring the participation of adherent cells to proliferate. The antigen is approximately 700,000 to 1x106 molecular weight and can be found in small quantities within some preparations of Johnin PPD. BoCD4+ cell lines reactive against this antigen fail to respond to M. avium or avian PPD (including strain 18). This antigen may have application in the identification of resistant animals, development of a specific subunit vaccine, and immunotherapy of infected animals.