A whole blood lymphocyte transformation test was used to examine cattle with varying degrees of infection with Mycobacterium paratuberculosis. Minimally infected animals characteristically responded to johnin purified protein derivative in the lymphocyte transformation test but did not routinely react on serological and/or skin testing. Heavily infected animals showed considerable variation in their lymphocyte transformation responses to antigen and some of them were consistently unresponsive. Antigen induced lymphocyte transformation reactions were recorded in 7.6 to 41.5% of uninfected animals whose infection status was determined by bacteriology and/or histopathology. The number of positive reactions recorded in uninfected animals depended on the population, the larger percentage being found in a herd with a proven history of paratuberculosis. The potential of lymphocyte transformation as a diagnostic test for bovine paratuberculosis is discussed.