['Nickel (Ni) is the major cause of contact allergy. We previously found that lipopolysaccharide (LPS, a cell-surface component of gram-negative bacteria) markedly promotes Ni allergy in a murine model. Establishing the minimum concentration or amount of Ni needed to induce allergic responses may help us to prevent or reduce such responses.', 'Using the above murine model, we examined the influence of LPS on the minimum allergy-inducing concentrations of Ni (Ni-MAICs) at the sensitization step and at the elicitation step.', 'BALB/c mice were sensitized by intraperitoneal injection of a mixture containing various concentrations of LPS and NiCl(2). Ten days later, their ear pinnas were challenged intradermally with a mixture containing various concentrations of LPS and NiCl(2), and ear swelling was measured.', 'Without LPS, the Ni-MAICs at the sensitization and elicitation steps were around 1×10(-2) mol L(-1) and 1×10(-5) mol L(-1) , respectively. Sensitization with NiCl(2) + LPS did not alter the value at elicitation. Surprisingly, LPS markedly reduced these Ni-MAICs (to around 1×10(-6) molL(-1) at sensitization, with 25 μg mL(-1) LPS, and 1×10(-12) mol L(-1) at elicitation, with 0·5 μg mL(-1) LPS). The effect of LPS depended on its concentration and the timing of its injection.'] Is allergy-inducing nickel concentration lowered by lipopolysaccharide at both the sensitization and elicitation steps in a murine model?