To study the role of food in the stimulation of cytokine production, the effects of lactic bacteria on production of interferon alpha, beta, and gamma; interleukin 1 beta; and tumor necrosis factor alpha were evaluated in mice and humans. Yogurt bacteria induced plasma interferon alpha and beta production in mice. Yogurt intake containing 10(11) bacteria led to increased 2'-5' A synthetase activity in human blood mononuclear cells. This result may suggest an interferon action in a peripheral way. This effect was also found when subjects consumed 10(8) yogurt bacteria/d for 15 d. In an in vitro model, blood mononuclear cells cultured in the presence of yogurt bacteria produced interleukin 1 beta, tumor necrosis factor, and interferon alpha and gamma. These results suggest the involvement of a certain type of food in cytokine production under healthy conditions.