Much of the research that has been done on the fish immune system has focussed on innate immunity. Very little is known about the adaptive immune system of fish and how it is regulated. This study has identified and characterised key cytokines that had not been found in any salmonids species to date, and that are potentially involved in T helper 1 (Th1) and Th17 type responses. The cloning of trout and salmon IL-1β3, and identification of a salmon IL-1β4 pseudogene revealed two types of IL-1β genes exist in teleost fish. Trout IL-1β3 is highly expressed in ovary suggesting a role in reproduction. A relatively high constitutive expression in gills, spleen and kidney and the up-regulation by PAMPs, proinflammatory cytokines and viral infection suggests IL-1β3 also has a role in inflammation and host defence. The IL-12 cytokine family are heterodimeric proteins that consist of an α-chain (p19, p28 or p35) and β-chain (p40 or EBI3). Due to the teleost wide whole genome duplication and/or salmonid whole genome duplication events, fish, especially salmonids, may have many paralogues of each subunit. Indeed, two distantly related p40 subunits termed p40A and p40B, as well as an EBI3 gene and a p35 gene have been previously cloned in rainbow trout. The cloning and sequence analysis of a p19 subunit gene was described in rainbow trout for the first time The expression and modulation of the (now known) five subunits (p19, p35, p40b, p40c and EBI3) of the trout IL-12 family members was comparatively examined in vivo in healthy fish and in fish after viral or parasite infection, and in vitro after stimulation with PAMPs, immune stimulants, suppressants, and recombinant trout cytokines. Bioactivity testing of two recombinant proteins of rainbow trout IL-12 paralogues was next studied. The recombinant IL-12A (p40c/p35a) and IL-12B (p40b/p35a) proteins were added to head kidney cultures and immune gene changes examined by real time PCR. This experiment 7 showed both up and down regulation of a number of the genes analysed, and revealed that the recombinant IL-12A and IL-12B proteins possess some shared bioactivities, but that some differences in function were also apparent. Fish in the Salmonidae family are characterized by having a relatively recent tetraploid ancestry, where a common ancestor of salmon and trout experienced whole genome duplication. Hence modern day species may be considered pseudo-tetraploid, as they are in the process of reverting to a stable diploid state. To gain further insight into the number of IL-12 family member paralogues present in salmonids, attention was turned to Atlantic salmon due to the availability of the initial genome sequencing contigs. Nine IL-12 cytokine family subunits (p19a, p19b, p35a1, p35a2, p35b1, p40c, p40b1, p40b2, and EBI3) were identified in Atlantic salmon and then comparatively examined in vivo in healthy fish and in fish after Poly I:C immune stimulation and in vitro after stimulation of head kidney cells with PAMPs, immune stimulants and suppressants, and recombinant trout cytokines. Having these genes available for study, along with what is currently known about the teleost immune system, will allow investigations into the adaptive immune responses of fish to a level not previously possible. Understanding these types of responses in fish and how they are regulated will help in the development of essential therapeutic strategies in fish.