The acute and sub chronic effects of oil sands process affected water (OSPW) and its toxic component, naphthenic acids (NAs) were examined. I exposed goldfish for one (acute), or 12 (sub chronic) weeks and examined immune gene expression, macrophage function, the ability of exposed fish to control infection with Trypanosoma carassii, and endocrine disruption. Acute exposure to 20 mg/L commercial naphthenic acids (C-NAs) or OSPW, induced up-regulation of the expression of immune genes. This up-regulation in gene expression was related increased resistance to T. carassii. After sub chronic exposures of goldfish to C-NAs or OSPW, decreased immune gene expression was observed, suggesting immunosuppression. This was confirmed by parasite-induced mortality to T. carassii. Acute exposure to fresh OSPW, and sub chronic exposure to aged or ozonated OSPW induced up-regulation in genes encoding endocrine receptors; suggesting possible long-term endocrine disruption following exposure to ozonated OSPW. Treatment with ozone ameliorated the acute immunotoxic effects of OSPW .The results in this thesis represent the first report of immunotoxicity caused by C-NAs or OSPW exposure in teleosts.