A battery of bioassays, including biological toxicity as well as in vitro mouse spleen lymphoproliferative responses and cytokine production, was conducted to compare the effectiveness of tertiary treatment methods such as coagulation (Coag) and absorption on granular activated carbon (GAC) and disinfection processes such as chlorination and ozonation in removing toxic or stress inducing agents from reclaimed wastewater. Whole effluent toxicity (WET) testing of secondary treated (ST) wastewater using as test species Vibrio fischeri, Daphnia magna and Tetrahymena thermophila as well as phytotoxicity revealed moderate toxicity effects that depend on the organism used. All bioassays exhibited decrease of the ecotoxicological responses after tertiary treatment. However, mitogenic responses were proved to be more sensitive. Endotoxin present in ST samples may be responsible for the increased strong lymphoproliferative activity as well as interleukin-1 (IL-1) production by mouse splenocytes. Tertiary treatment of ST with coagulation and/or adsorption on granular activated carbon (GAC) in combination with ozonation reduced WET to control levels. Ozonation alone or in combination with any other treatment removed endotoxin more efficiently than chlorination and thus reduced spleen lymphoproliferative responses and IL-1 production.