Provided are: an anti-protein-toxin (anti-endotoxin and/or anti-enterotoxin) agent for livestock and fish, containing smectite as an active ingredient; and a method for preventing the action of a protein toxin on livestock through oral administration of the anti-protein toxin agent. Said smectite is preferably a calcium-type smectite, a sodium-type smectite, and an activated smectite.