AIM: To investigate the capacity of potentially probiotic strains from six bacterial genera to induce cytokine production alone or in combinations in order to identify potential enhancing or synergistic effects in order to select probiotic bacteria for in vivo purposes. METHODS: Cytokine production in human peripheral blood mononuclear cells (PBMC) in response to stimulation with eleven different potentially probiotic bacterial strains from Streptococcus, Lactobacillus, Bifidobacterium, Lactococcus, Leuconostoc and Propionibacterium genera was analysed. Production and mRNA expression of TNF-α, IL-12,IFN-γ and IL-10 were determined by ELISA and Northern blotting, respectively. RESULTS: All tested bacteria induced TNF-α production. The best inducers of Thl type cytokines IL-12 and IFN-γ were Streptococcus and Leuconostoc strains. All Bifidobacteriurn and Propionibacteriurn strains induced higher IL-10 production than other studied bacteria. Stimulation of PBNC with any bacterial combinations did not result in enhanced cytokine production suggesting that different bacteria whether gram-positive or gram negative compete with each other during host cell interactions. CONCLUSION: The probiotic S. thermophilus and Leuconostoc strains are more potent inducers of Thl type cytokines IL-12 and IFN-γ than the probiotic Lactobacillus strains. Bacterial combinations did not result in enhanced cytokine production.