Abstract Heavy metals such as zinc, copper, iron and lead, when discharged in mixtures into water, produce an entirely different lethal impact on aquatic life than when they are discharged separately. The toxicity of these metals in mixtures to zooplankton ( Diaptomus forbesi) was increased by 39.2% in the presence of the detergent parnol-J. The presence of a petroleum product, chlorinated and organophosphorus pesticides and nitrogen and phosphorus fertilizers increased the toxicity of metal mixtures to planktons by about 80%. When the petroleum product n-hexane was mixed with the metals, the lethality to worms ( Branchiura sowerbyi) was decreased by 80%, but when the detergent parnol-J was drained with the metals, the lethality was decreased by 42%. Different combinations of metals such as Zn-Cu-Fe, Zn-Cu-Pb, Zn-Fe-Pb, Cu-Fe-Pb, and Zn-Cu-Fe-Pb showed synergism in the receiving water. The synergistic action of different pollutant mixtures proves that the conventional safe disposal method (based on one-third to one-sixth of LC 50 levels of individual pollutants) is no longer safe for aquatic life. Therefore the concept of safe disposal should be based on the non-lethal doses of specific mixtures of toxicants.