The T-cell subsets taking part in the responses of eight calves infected with a caprine isolate of M. a. paratuberculosis were studied. Blood and faeces were sampled at regular intervals to monitor the cells responding to PPDp in vitro and to detect faecal excretion of M. a. paratuberculosis. The IFN-gamma response of whole blood to in vitro PPDp stimulation was higher in all the infected animals than in a group of age matched control animals from three months post infection and onwards. Six of the eight infected calves excreted bacteria to faeces during the third and fourth month post infection. The CD4+ cells were the major IFN-gamma producers as shown by intracellular staining. However, in addition to CD4+ cells, both CD8+ and gamma delta TcR+ cells from the infected animals up regulated IL-2 receptor (IL-2R) in response to in vitro PPDp stimulation. Of the gamma delta TcR+ subpopulations, mainly the WC1+ cells have been studied in mycobacterial infections of ruminants. However, as the WC1-CD2+ cells are the most abundant gamma delta TcR+ subpopulation in the intestine, the functions of these cells were studied further. While the WC1+ gamma delta TcR+ cells had a high level of IL-2R in peripheral blood, which was sustained by PPDp stimulation in vitro, the CD2+ gamma delta TcR+ cells had lower levels of IL-2R in peripheral blood, but up-regulated the IL-2R expression through in vitro PPDp stimulation. These results indicate that the CD2+ and the WC1+ gamma delta T-cells have different kinetics of activation and may play different roles in the immune response against mycobacteria.