The ability to select for a cell-mediated response rather than antibody production following infection with intracellular mycobacteria, would be an advantage in preventing the occurrence of disease. Recent work suggest that the two members of the B7 family of costimulatory molecules, B7-1 and B7-2, may differentially influence the nature of primary immune responses but little is known of their role in this capacity in secondary responses. We have used an in vitro model to investigate whether blocking B7-1 and B7-2 affects changes in the cytokine profiles of Th lymphocytes previously primed to purified protein derivative (PPD) from Mycobacterium bovis. In C57BL/6 and BALB/c mice we found that the proliferative responses of a component of recently activated T lymphocytes, and those returning to the resting state, were inhibited by B7-2 blockade. B7-1 blockade had no distinguishable effect. However, in cultures containing anti-B7-2 antibody, the production of both interferon-γ (IFN-γ) and interleukin-4 (IL-4), indicative of cell-mediated and antibody responses, respectively, were induced. This suggests that intervention in a recall response to mycobacterial antigen by blocking B7-1 or B7-2 molecules, is unlikely to alter the nature of the immune response.