Juvenile coho salmon (Oncorhynchus kisutch), vaccinated with one intraperitoneal injection of formalin-killed virulent Aeromonas salmonicida cells suspended in saline, showed increased protection against approximately one LD60 of homologous challenge administered at 30 days post-vaccination. Under similar conditions, coho vaccinated with a modified complete Freund's adjuvant (MFCA) alone were also equally protected. When measured against a more severe A. salmonicida challenge of approximately one LD95, the strength of the MFCA-induced protection was found to exceed that produced by the homologous bacterin administered in saline or incomplete adjuvant, and the protection was still evident at 90 days post-treatment. Other more precise measurements indicated the LD50 for MFCA-treated coho to be up to 450 times that for saline-treated coho. Two other tested adjuvants, levamisole and MDP (N-acetyl-muramyl-L-alanyl-D-isoglutamine), administered in a modified Freund's incomplete adjuvant, also enhanced anti-A. salmonicida immunity but to a lesser degree. The active factor in MFCA was a killed Mycobacterium butyricum preparation, and the anti-A. salmonicida immunity it induced was non-specific because the immunity extended to two other serologically distinct fish pathogens tested: A. hydrophila (LD50 increase of 5.3-fold) and Vibrio ordalii (LD50 increase of 560-fold). Macrophages are believed to account for the M. butyricum-induced anti-A. salmonicida immunity because the immunity was a) non-specific, b) very rapid in onset (it was measurable by 4 days), and c) influenced by particulate preparations, known to affect macrophage function and immunity in mammals. The possible benefits of adjuvant-induced non-specific immunity in cultured fish are discussed.