In the environment, Gram-negative bacteria are capable of producing large amounts of endotoxin-containing microvesicles – spherical structures measuring 30–50 nm in diameter, emerging by fragmentation of the outer membrane of the bacterial cell wall. Microvesicles are suspected of inducing inflammatory lung diseases, but possibly also of stimulating anti-tumour defence mechanisms. The present study was aimed at assessing the pro-inflammatory and anti-tumour properties of microvesicles in vitro. Peripheral blood mononuclear cells of 5 healthy volunteers were cultured for 6 h, 24 h, 3 days, and 5 days with microvesicles (MV) of Pantoea agglomerans at concentrations ranging from 0.48–1500 μg/ml. The following outcomes were measured: secretion of IFN-γ and TNF-α (by ELISA and ELISpot), intensity of cell proliferation (LPT), expression of surface markers CD8, CD14, CD16, CD25, CD69, CD80, CD83, HLA-DR, and apoptosis markers (by flow cytometry). After 24 hours, a clear dose-dependent response to microvesicles was seen for IFN-γ production, starting already at the lowest concentration of 0.48 μg/ml (p=0.04). A 2-fold increase in TNF-α production was seen after 3 days at the concentration of 1,500 μg/ml (p=0.05). A clear and signifi cant dose-dependent increase in cell proliferation in response to MV was detectable after 5 days (p=0.001). A decrease in the percentage of CD14(+)CD83(+) monocytes was observed after 1 day of culture. We conclude that IFN-γ and TNF-α are triggered at different concentrations of microvesicles: at lower concentrations only IFN-γ is upregulated, whereas at higher concentrations both IFN-γ and TNF-α are secreted.