Endotoxin (Et) can be specifically assayed by exclusively utilizing the factor C system reaction without being affected by factor G contained in a limulus amebocyte lysate reagent. The present invention provides: (1) a reagent for ET-specific assay which comprises a limulus amebocyte lysate reagent and an alkylglucoside; (2) a method of specifically assaying Et in a specimen using a limulus amebocyte lysate reagent, wherein an alkylglucoside is added to the limulus amebocyte lysate reagent and/or the specimen; and (3) a factor G activation inhibitor composition which comprises an alkylglucoside as an active ingredient capable of inhibiting the activation of factor G in limulus amebocyte by (1.fwdarw.3)-.beta.-D-glucan.