The purpose of this study was to characterise biotransformation and immune responses in juvenile Atlantic cod (Gadus morhua) following exposure for 2, 8 and 16 days to the water-soluble fraction of North Sea crude oil in different concentrations. Biomarkers (ELISA, hepatic EROD activity and PAH metabolites in bile) were used to examine effects. The study also included oil-related effects on DNA by assessing DNA damage with the comet assay. A third aspect of the study was quantifying immune responses related to oil exposure, which was done by measuring respiratory burst in leukocytes. A pathogen challenge in the form of LPS was injected after 8 days to assess if the immunocompetence changed as a result of oil exposure. The biomarkers of exposure were successful in linking oil treatments with elevated PAH metabolite levels, EROD activity and a change in CYP1A levels. There was no link between increased DNA damage and exposure to the WAF of oil. Furthermore, there were no treatment effects on respiratory burst. Injection of LPS did not appear to change immunocompetence. In total, the study provided good insight into effects of the WAF of crude oil on biotransformation pathways in cod as well as assessing immune responses, although the latter may need further studies to successfully contribute to risk assessment.