Aminobisphosphonates (aminoBPs) are potent inhibitors of bone resorption. However, they cause undesirable inflammatory reactions, including fever, in humans. Intraperitoneal injection of aminoBPs into mice also induces inflammatory reactions, including a prolonged elevation of the activity of the histamine-forming enzyme, histidine decarboxylase (HDC). Because interleukin-1 (IL-1) is a typical pyrogen and a strong inducer of HDC, we examined whether aminoBPs induce inflammatory reactions in mice deficient in genes for both IL-1alpha and IL-1beta (IL-1-KO mice). In control mice, aminoBPs induced an elevation of HDC activity and other inflammatory reactions (enlargement of the spleen, atrophy of the thymus, exudate in the thorax and increase in granulocytic cells in the peritoneal cavity). These responses were all weak or undetectable in IL-1-KO mice. We have previously shown that lipopolysaccharides (LPSs) from Escherichia coli and Prevotella intermedia (a prevalent gram-negative bacterium both in periodontitis and endodontal infections) are capable of inducing HDC activity in various tissues in mice. In control mice treated with an aminoBP, the LPS-induced elevations of serum IL-1 (alpha and beta) and tissue HDC activity were both markedly augmented. However, such an augmentation of HDC activity was small or undetectable in IL-1-KO mice. These results, taken together with our previous findings (i) suggest that IL-1 is involved in the aminoBP-induced inflammatory reactions and (ii) lead us to think that under some conditions, inflammatory reactions induced by gram-negative bacteria might be augmented in patients treated with an aminoBP. In this study, we also obtained a result suggesting that IL-1-deficiency might be compensated by a second, unidentified, mechanism serving to induce HDC in response to LPS when IL-1 is lacking.