The immune responses elicited by CpG ODN and DNA vaccines in rainbow trout were studied, both in vitro and in vivo.  The results indicated that CpG ODN are capable of upregulating the expression of various immune genes and genes involved in the bactericidal ability of rainbow trout leucocytes.  They also enhance the bactericidal ability of head kidney in vitro although they are unable to enhance or induce respiratory burst in these cells.  Additionally, CpG ODN are capable of enhancing leucocyte migration, both in vitro and in vivo.  Humoral immune parameters were rather un-responsive to i.p. injection of CpG ODN with serum lysozyme activity only being increased in fish injected with 10mg CpG (the highest dose tested) 10 days post injection while neither serum antiprotease nor classical complement activity were affected over the time period studied.  Despite their inability to upregulate antibody specific production following i.p. injection, CpG ODN did exhibit a significant, dose dependent effect on lymphocyte proliferation.  Specific CpG ODN also significantly enhanced protection against challenge with Aeromonas salmonicida both when used as an adjuvant to a commercially available vaccine and when administrated alone in PBS.  With regard to the effect of DNA vaccination on virus-specific, cell-mediated cytotoxicity, the results obtained here were inconclusive with further work being required in this field before any conclusions can be drawn.