Specific antibody production in response to an intraperitoneal injection of A. salmonicida endotoxin first occurred in juvenile coho salmon (Oncorhynchus kisutch) held at 6.7, 12.2, and 17.8 C at 4, 2 and 1 wk, respectively. Aeromonas salmonicida endotoxin was antigenic to juvenile coho salmon at all doses between 1 and 5000 μg administered intraperitoneally with and without an aluminum hydroxide carrier.The endotoxin was nontoxic to juvenile coho salmon when 1–5000 μg was injected intraperitoneally and the fish held at temperatures from 6.7 to 17.8 C or when "free endotoxin" was administered by the intramuscular or intraperitoneal route. Aeromonas salmonicida endotoxin was toxic to mice, however.