Lymphocyte blastogenesis, monocyte migration inhibition and cutaneous hypersensitivity were measured in cattle with chronic paratuberculosis before and after sensitization with M. bovis, M. paratuberculosis, and keyhole limpet hemocyanin in Freund's incomplete adjuvant. The greatest in vitro responses were seen with M. bovis antigen and no differences were detected pre- and post-sensitization with these assays. Pre-sensitization skin testing with M. bovis PPD and johnin was variable with a significant decrease in cutaneous responses after sensitization with the mycobacterial antigens. In vitro and in vivo reactivity to keyhole limpet hemocyanin was negligible in infected cattle both before and after sensitization with KLH. These results indicate that cattle with paratuberculosis respond suboptimally to heterologous antigen sensitization and are prone to desensitization when homologous antigens are administered.