Unacceptable risks are posed to ecological receptors from chemicals as well as many other pollutants. Chemical contamination is considered a major ecological stress factor on fresh water fish. Stress can cause a reduction in feed intake, impair growth, reproduction and immunity. Copper sulfate, Malathion and Paraquat, as industrial overuse and agricultural pesticides, can cause extensive contamination to aquatic environments. This study is concerned with the determination of the modulating effect of acute sublethal doses of the three chemicals in the fresh water fish rainbow trout (Onchorynchus mykiss). A monoclonal antibody dependent indirect enzyme linked immunosorbant assay (ELISE) was used. The evaluation of the immune modulation was based on the determination the IgM titers in correlation to KLH immunization. Significant changes in the IgM patterns indicated suppression and/or enhancement of the immune system of fish. Such data could be of value for preliminary problem formulation and ecological effects evaluation in ecosystems.