Abstract: Formalin-killed, heat-killed and lipopolysaccharide vaccines against Aeromonas hydrophila and a
bivalent formalin-killed vaccine against A. hydrophila and A. veronei bv. sobria were tested in rainbow trout
(Oncorhynchus mykiss). The evaluation of trout fish immune response after vaccination with Aeromonads
bacterins by immersion and bath challenge route was undertaken using an indirect enzyme-linked
immunosorbent assay (ELISA).To test the strength of protection, the challenge process was examined using
10 cells of the live bacteria/ml of A. hydrophila. The results showed that the relative 5 percentage of survival in
the trout fish groups vaccinated by heat-killed type of vaccine were significantly higher (P