An endotoxin stabilizing agent is provided, which is useful in order to maintain the endotoxin activity of a specimen or a reference standard in a stable state for a prolonged period of time and to prepare an endotoxin reference standard having a reduced intervial variation, being stable for a long time in the form of, in particular, a solution and withstanding repeated use. An endotoxin composition comprising the above-mentioned endotoxin stabilizing agent and endotoxin and a method for assaying endotoxin by using the same are also provided. The endotoxin stabilizing agent comprises: (1) at least one substance selected from the group consisting of aminoalcohols, polyhydric alcohols, nonionic surfactants, polysucroses and chelating agents; (2) at least aminoalcohol and polyhydric alcohol; (3) with the polyhydric alcohol being preferably glycerol or its derivative; (4) polyethylene glycol as an effective ingredient in addition to the components (1) to (3); or (5) at least polysucrose, and an alkaline earth metal salt and/or polyethylene glycol as an effective ingredient. The endotoxin composition comprises the above-identified endotoxin stabilizing agent. The method for assaying endotoxin comprises adding the above-described endotoxin stabilizing agent to a specimen to thereby stabilize the activity of endotoxin in the specimen.