Causes of individual variation in susceptibility to mycobacterial diseases are only partly understood. An efficient cell-mediated immune response is crucial for resistance. Macrophages and T cells interact to eliminate the mycobacteria, partially through the effects of secreted cytokines. A vigorous anti-bacterial inflammatory response is sometimes accompanied by severe tissue damage, while immunosuppression leads to progressive infection. Here, live, attenuated Mycobacterium bovis, bacillus Calmette-Guérin (BCG), was used as a model antigen to study cytokine production at the single-cell level in response to mycobacteria. Peripheral blood mononuclear cells from healthy individuals were challenged in vitro and the kinetics and frequencies of cytokine-producing cells were studied by immunofluorescent visualization of intracellular cytokines. Fourteen cytokines were assayed; interleukin-1 alpha (IL-1 alpha), IL-1 beta, IL-1 receptor antagonist (IL-1ra), IL-2, IL-3, IL-4, IL-5, IL-6, IL-8, IL-10, interferon-gamma (IFN-gamma), tumour necrosis factor-alpha (TNF-alpha), TNF-beta and granulocyte-macrophage colony-stimulating factor (GM-CSF). A sequential production of T helper-1 (Th1) and T helper-2 (Th2) cytokines was induced by BCG. Early, at days 1-2 after stimulation, the response was dominated by monokines and a low IFN-gamma and TNF-beta production. At days 4-5 there was a marked production of Th1 lymphokines, with approximately 6% IFN-gamma+ cells, 4% TNF-beta+ cells and 2% IL-2+ cells. Late in the reaction, at days 10-12, a Th2 response with IL-4, IL-5 and IL-10 was detected, while the synthesis of Th1 lymphokines and monokines declined. Overall, our results provide further evidence of IFN-gamma as the major cytokine induced by mycobacteria in healthy individuals, but also suggest that Th2 cytokines participate in the response.