Interaction of macrophages with bacteria is a stimulus for production of cytokines such as IL-10 and IL-12. IL-12 stimulates T cell and natural killer (NK) cell cytotoxicity and interferon-gamma (IFN-γ) production. IL-10 opposes the T cell-stimulating action of IL-12, decreases the release of proinflammatory cytokines from macrophages, and stimulates B cells. We have studied the capacity of human intestinal isolates from the three Lactobacillus species dominating on the human gastrointestinal mucosa, L. plantarum, L. rhamnosus and L. paracasei ssp. paracasei, to induce production of IL-10 and IL-12 from human blood mononuclear cells, or monocytes. Whole killed lactobacilli were potent stimulators of IL-12 over a wide range of bacterial concentrations. Lactobacillus paracasei gave the highest levels of IL-12 (1.5 ng/ml in response to 5 × 106 bacteria/ml), roughly 10 times more than obtained by stimulation with L. rhamnosus or L. plantarum. Escherichia coli induced on average < 50 pg/ml of IL-12 regardless of the bacterial concentration used. The secretion of free p40 subunit IL-12 followed the same pattern as the secretion of p70 (bioactive IL-12) with regard to the efficiency of different bacteria as stimulators. Escherichia coli was the most efficient trigger of IL-10 production, inducing 0.5 ng/ml IL-10 after stimulation with 5 × 106 bacteria/ml. Lactobacillus rhamnosus induced the highest levels of IL-10 among the lactobacilli (0.5 ng/ml) compared with 0.1 ng/ml evoked by L. plantarum or L. paracasei, but 10 times more bacteria were required for optimal stimulation than with E. coli. When neutralizing anti-IL-10 antibodies were added to the cultures, the IL-12-inducing capacity of L. rhamnosus was increased markedly, while that of E. coli remained low. The results show that mucosa-associated lactobacilli can be potent stimulators of IL-12, and thus potentially of cell-mediated immunity, if they pass over the gut epithelial barrier and interact with cells of the gut immune system.