This paper describes the field evaluation of a serological test and a new in vitro assay for cell-mediated reactivity for the diagnosis of bovine tuberculosis. The use of a Mycobacterium bovis-specific antigen (MPB-70) in an ELISA to test the serological response to tuberculosis infection resulted in a specificity of 96.4% and a sensitivity of 18.1%. The most favourable results were obtained with the interferon gamma (IFN-gamma) assay which had a sensitivity of 81.8% and a specificity of 99.1%. Respective figures for the single intradermal tuberculin test were 68.1% and 96.7%. The use of MPB-70 as the antigen in the IFN-gamma assay reduced the sensitivity of this assay, without producing any useful increase in specificity. The IFN-gamma assay was also demonstrated to be a practical diagnostic test for use with large groups of cattle.