This invention provides (1) a reagent for endotoxin assay which comprises aprotinin and a limulus amebocyte lysate reagent, (2) a kit for endotoxin assay which comprises the limulus amebocyte lysate reagent and a reagent containing aprotinin, (3) a method for assaying endotoxin in a sample using the limulus amebocyte lysate reagent in which aprotinin is added to the lysate reagent and/or the sample, (4) a method for assaying endotoxin in a serine protease-containing sample using the limulus amebocyte lysate reagent in which the sample is allowed to contact with an aprotinin-immobilized insoluble carrier in advance of endotoxin assay, (5) a carrier for pretreating a serine protease-containing sample on which aprotinin is immobilized, (6) a method for inhibiting factor G activation in which aprotinin is added to the limulus amebocyte lysate reagent and (7) a factor G activation inhibitor which comprises aprotinin as an active ingredient. Endotoxin assay can be effected based on the factor C system reaction, without influences of factor G contained in the limulus amebocyte lysate reagent and/or serine proteases contained in samples.