The immunosuppressive effects of pollutants from industry (effluents, heavy metals) and agriculture (pesticides) on freshwater fish, as a sentinel model for the aquatic environment, are presented in this paper. An impairment of humoral, cellular and/or non‐specific immunity compromises the defence mechanisms against pathogens. For ten years, we studied the effects on rainbow trout and carp of pesticides such as organochlorine (lindane) and organophosphorous (trichlorfon, dichlorvos) insecticides, and a triazine herbicide (atrazine); heavy metals (Cu, Zn, Mg, Mn, Hg) were also analysed for theirin vitroeffects. More recently, pulp mill effluents were studied on rainbow trout immunity. Most of those aquatic pollutants were shown to be partial or total immunosuppressors of the major functions ofsomefreshwater fish immune system.