In this study, interferon-gamma (IFN-gamma) responses in whole blood cultures stimulated with tuberculins from different sources were compared with regard to their diagnostic reliability in cattle experimentally and naturally infected with Mycobacterium bovis. The IFN-gamma responses to different concentrations of purified protein derivatives (PPDs) from M bovis and Mycobacterium avium were quantified. Significant differences (P<0.05) between sources and concentrations of PPDs used for stimulation were detected, indicating a need for standardisation of PPDs used in the IFN-gamma assay. Additionally, a tool named'relative potency 30' that allows rapid comparison of batches and sources of PPDs was defined.