Exacerbation of antibody-mediated thrombocytopenia following infection with viruses has recently been demonstrated in a mouse model of the disease. The phenomenon was caused by an increased activation of phagocytes through gamma-interferon secretion in response to infection. Endotoxins from Gram-negative bacteria are also known to be potent activators of phagocytic cells. The objective of the present work was to determine whether lipopolysaccharide (LPS) could exacerbate antibody-mediated thrombocytopenia in vivo and so alter the therapeutic efficacy of intravenous immunoglobulin (IVIg), using a mouse model of thrombocytopenia. Very low doses of LPS (picogram range) and of anti-platelet antibodies (nanogram range), which did not induce thrombocytopenia individually, could synergize in vivo, resulting in significant decreases in platelet counts. The therapeutic efficacy of IVIg in antibody-mediated thrombocytopenia was significantly reduced in presence of LPS. These in vivo observations further support a role for bacterial infections in the aetiology of immune thrombocytopenic purpura (ITP) and may contribute to better understand the recognized lack of efficacy of IVIg in a significant proportion of patients with ITP.